# Organic Chemistry/Chirality

## Introduction

**Chirality** (pronounced kie-RAL-it-tee) is the property of
*handedness*. If you attempt to superimpose your right hand on top of
your left, the two do not match up in the sense that your right hand\'s
thumb overlays your left hand\'s pinky finger. Your two hands cannot be
superimposed identically, despite the fact that your fingers of each
hand are connected in the same way. Any object can have this property,
including molecules.

An object that is **chiral** is an object that can not be superimposed
on its mirror image. Chiral objects don\'t have a *plane of symmetry*.
An achiral object has a plane of symmetry or a rotation-reflection axis,
i.e. reflection gives a rotated version.

**Optical isomers** or **enantiomers** are stereoisomers which exhibit
chirality. Optical isomerism is of interest because of its application
in inorganic chemistry, organic chemistry, physical chemistry,
pharmacology and biochemistry.

They are often formed when asymmetric centers are present, for example,
a carbon with four different groups bonded to it. Every stereocenter in
one enantiomer has the opposite configuration in the other.

When a molecule has more than one source of asymmetry, two optical
isomers may be neither perfect reflections of each other nor
superimposeable: some but not all stereocenters are inverted. These
molecules are an example of **diastereomers**: They are not enantiomers.
Diastereomers seldom have the same physical properties. Sometimes, the
stereocentres are themselves symmetrical. This causes the
counterintuitive situation where two chiral centres may be present but
no isomers result. Such compounds are called **meso compounds**.

!Chiral
relationship.{width="300"}

A mixture of equal amounts of both enantiomers is said to be a **racemic
mixture**.

It is the symmetry of a molecule (or any other object) that determines
whether it is chiral or not. Technically, a molecule is achiral (not
chiral) if and only if it has an axis of improper rotation; that is, an
n-fold rotation (rotation by 360°/n) followed by a reflection in the
plane perpendicular to this axis which maps the molecule onto itself. A
chiral molecule is not necessarily dissymmetric (completely devoid of
symmetry) as it can have, e.g., rotational symmetry. A simplified rule
applies to tetrahedrally-bonded carbon, as shown in the illustration: if
all four substituents are different, the molecule is chiral.

It is important to keep in mind that molecules which are dissolved in
solution or are in the gas phase usually have considerable flexibility
and thus may adopt a variety of different conformations. These various
conformations are themselves almost always chiral. However, when
assessing chirality, one must use a structural picture of the molecule
which corresponds to just one chemical conformation -- the one of lowest
energy.

## Chiral Compounds With Stereocenters

Most commonly, chiral molecules have point chirality, centering around a
single atom, usually carbon, which has four different substituents. The
two enantiomers of such compounds are said to have different absolute
configurations at this center. This center is thus stereogenic (i.e., a
grouping within a molecular entity that may be considered a focus of
stereoisomerism), and is exemplified by the α-carbon of amino acids.

The special nature of carbon, its ability to form four bonds to
different substituents, means that a mirror
image of the carbon with four different bonds will not be the same as
the original compound, no matter how you try to rotate it. Understanding
this is vital because the goal of organic chemistry is understanding how
to use tools to synthesize a compound with the desired chirality,
because a different arrangement may have no effect, or even an undesired
one.

A carbon atom is chiral if it has four different items bonded to it at
the same time. Most often this refers to a carbon with three heteroatoms
and a hydrogen, or two heteroatoms plus a bond to another carbon plus a
bond to a hydrogen atom. It can also refer to a nitrogen atom bonded to
four different types of molecules, if the nitrogen atom is utilizing its
lone pair as a nucleophile. If the nitrogen has only three bonds it is
**not** chiral, because the lone pair of electrons can flip from one
side of the atom to the other spontaneously.

*Any atom in an organic molecule that is bonded to four different types
of atoms or chains of atoms can be considered \"chiral\".*

If a carbon atom (or other type of atom) has four different
substituents, that carbon atom forms a *chiral center* (also known as a
*stereocenter*). Chiral molecules often have one or more stereocenters.
When drawing molecules, stereocenters are usually indicated with an
asterisk near the carbon.

**Example:**

**Left**: The carbon atom has a Cl, a Br, and 2 CH~3~. That\'s only 3
different substituents, which means this is not a stereocenter.\
**Center**: The carbon atom has one ethyl group (CH~2~CH~3~), one methyl
group (CH~3~) and 2 H. This is not a stereocenter.\
**Right**: The carbon atom has a Cl and 1 H. Then you must look around
the ring. Since one side has a double bond and the other doesn\'t, it
means the substituents off that carbon are different. The 4 different
substituents make this carbon a stereocenter and makes the molecule
chiral.

A molecule can have multiple chiral centers without being chiral
overall: It is then called a meso compound. This occurs if there is a
symmetry element (a mirror plane or inversion center) which relates the
chiral centers.

### Fischer projections

Fischer projections (after the German chemist Hermann Emil
Fischer) are an ingenious means for
representing configurations of carbon atoms. Considering the carbon atom
as the center, the bonds which extend towards the viewer are placed
horizontally. Those extending away from the viewer are drawn vertically.
This process, when using the common dash and wedge representations of
bonds, yields what is sometimes referred to as the \"bowtie\" drawing
due to its characteristic shape. This representation is then further
shorthanded as two lines: the horizontal (forward) and the vertical
(back), as showed in the figure below:

!**Principle of Fischer
projection**

#### Operations on Fischer projections

-   *in a Fischer projection, exchange two substituent positions results
    in the inversion of the stereocenter*
-   *rotation by 90° of the Fischer projection results in inversion*
-   *rotation by 180° of the Fischer projection preserves the
    configuration*

## Naming conventions

There are three main systems for describing configuration: the oldest,
the *relative* whose use is now deprecated, and the current, or
*absolute*. The relative configuration description is still used mainly
in glycochemistry. Configuration can also be assigned on the purely
empirical basis of the optical activity.

### By optical activity: (+)- and (-)-

An optical isomer can be named by the direction in which it rotates the
plane of polarized light. If an isomer rotates the plane clockwise as
seen by a viewer towards whom the light is traveling, that isomer is
labeled (+). Its counterpart is labeled (-). The (+) and (-) isomers
have also been termed d- and l-, respectively (for dextrorotatory and
levorotatory). This labeling is easy to confuse with D- and L- and is
therefore not encouraged by IUPAC.

The fact that an enantiomer can rotate polarised light clockwise (*d*-
or *+*- enantiomer) does not relate with the relative configuration (D-
or L-) of it.

### By relative configuration: D- and L-

Fischer, whose research interest was in carbohydrate chemistry, took
glyceraldehyde (the simplest sugar, systematic name
2,3-dihydroxypropanal) as a template chiral molecule and denoted the two
possible configurations with D- and L-, which rotated polarised light
clockwise and counterclockwise, respectively.

!Glycerladehyde, the starting molecule for *relative configuration*
assignent
{width="450"}

All other molecules are assigned the D- or L- configuration if the
chiral centre can be formally obtained from glyceraldehyde by
substitution. For this reason the D- or L- naming scheme is called
*relative configuration*.

+----------------------------------------------------------------------+
|   ---------------------------------------                            |
| ------------------------------------------------ ------------------- |
| -------------------------------------------------------------------- |
|                                                                      |
|                                 D-glyceraldehyde                     |
|                                                     L-glyceraldehyde |
|     !D-glyceraldehyde{width="80"}    !L-glyceraldehy |
| de{width="100"} |
|    !D-glyceraldehyde{width="150"}   !L-glyceraldehyd |
| e{width="150"} |
|     !D-glyceraldehyde{width="150"}      !L-glyceralde |
| hyde{width="150"} |
|   ---------------------------------------                            |
| ------------------------------------------------ ------------------- |
| -------------------------------------------------------------------- |
+----------------------------------------------------------------------+

An optical isomer can be named by the spatial configuration of its
atoms. The D/L system does this by relating the molecule to
glyceraldehyde. Glyceraldehyde is chiral itself, and its two isomers are
labeled D and L. Certain chemical manipulations can be performed on
glyceraldehyde without affecting its configuration, and its historical
use for this purpose (possibly combined with its convenience as one of
the smallest commonly-used chiral molecules) has resulted in its use for
nomenclature. In this system, compounds are named by analogy to
glyceraldehyde, which generally produces unambiguous designations, but
is easiest to see in the small biomolecules similar to glyceraldehyde.

!Optical isomers{width="300"} One
example is the amino acid alanine: alanine has two optical isomers, and
they are labeled according to which isomer of glyceraldehyde they come
from. Glycine, the amino acid derived from glyceraldehyde, incidentally,
does not retain its optical activity, since its central carbon is not
chiral. Alanine, however, is essentially methylated glycine and shows
optical activity.

The D/L labeling is unrelated to (+)/(-); it does not indicate which
enantiomer is dextrorotatory and which is levorotatory. Rather, it says
that the compound\'s stereochemistry is related to that of the
dextrorotatory or levorotatory enantiomer of glyceraldehyde. Nine of the
nineteen L-amino acids commonly found in proteins are dextrorotatory (at
a wavelength of 589 nm), and D-fructose is also referred to as levulose
because it is levorotatory.

The dextrorotatory isomer of glyceraldehyde is in fact the D isomer, but
this was a lucky guess. At the time this system was established, there
was no way to tell which configuration was dextrorotatory. (If the guess
had turned out wrong, the labeling situation would now be even more
confusing.)

A rule of thumb for determining the D/L isomeric form of an amino acid
is the \"CORN\" rule. The groups:

:   COOH, R, NH2 and H (where R is an unnamed carbon chain)

are arranged around the chiral center carbon atom. If these groups are
arranged clockwise around the carbon atom, then it is the L-form. If
counter-clockwise, it is the D-form.This rule only holds when the
hydrogen atom is pointing out of the page.[^1]

### By absolute configuration: R- and S-

Main article: R-S
System

The absolute configuration system stems from the Cahn-Ingold-Prelog
priority rules,
which allow a precise description of a stereocenter without using any
reference compound. In fact the basis is now the atomic number of the
stereocenter substituents.

The R/S system is another way to name an optical isomer by its
configuration, without involving a reference molecule such as
glyceraldehyde. It labels each chiral center R or S according to a
system by which its ligands are each assigned a priority, according to
the Cahn Ingold Prelog priority rules, based on atomic number.

This system labels each chiral center in a molecule (and also has an
extension to chiral molecules not involving chiral centers). It thus has
greater generality than the D/L system, and can label, for example, an
(R,R) isomer versus an (R,S) --- diastereomers.

The R/S system has no fixed relation to the (+)/(-) system. An R isomer
can be either dextrorotatory or levorotatory, depending on its exact
ligands.

The R/S system also has no fixed relation to the D/L system. For
example, one of glyceraldehyde\'s ligands is a hydroxy group, -OH. If a
thiol group, -SH, were swapped in for it, the D/L labeling would, by its
definition, not be affected by the substitution. But this substitution
would invert the molecule\'s R/S labeling, due to the fact that
sulfur\'s atomic number is higher than carbon\'s, whereas oxygen\'s is
lower. \[Note: This seems incorrect. Oxygen has a higher atomic number
than carbon. Sulfur has a higher atomic number than oxygen. The reason
the assignment priorities change in this example is because the CH2SH
group gets a higher priority than the CHO, whereas in glyceraldehyde the
CHO takes priority over the CH2OH.\]

For this reason, the D/L system remains in common use in certain areas,
such as amino acid and carbohydrate chemistry. It is convenient to have
all of the common amino acids of higher organisms labeled the same way.
In D/L, they are all L. In R/S, they are not, conversely, all S --- most
are, but cysteine, for example, is R, again because of sulfur\'s higher
atomic number.

The word "racemic" is derived from the Latin word for grape; the term
having its origins in the work of Louis Pasteur who isolated racemic
tartaric acid from wine.

## Chiral Compounds Without Stereocenters

It is also possible for a molecule to be chiral without having actual
point chirality (stereocenters). Commonly encountered examples include
1,1\'-bi-2-naphthol (BINOL) and 1,3-dichloro-allene which have axial
chirality, and (E)-cyclooctene which has planar chirality.

For example, the isomers which are shown by the following figure are
different. The two isomers cannot convert from one to another
spontaneously because of restriction of rotation of double bonds.\
![](Hexa-3,4-dien-3-ol.png "Hexa-3,4-dien-3-ol.png")\
Other types of chiral compounds without stereocenters (like restriction
of rotation of a single bond because of steric hindrance) also exist.
Consider the following example of the R and S binol molecules:

![]((R)-_and_(S)-BINOL.svg "(R)-_and_(S)-BINOL.svg"){width="200"}

![](Chiral_biphenyl.svg "Chiral_biphenyl.svg"){width="150"}

*The biphenyl C-C bond cannot rotate if the X and Y groups cause steric
hindrance.*

![](Spiroverbindung_Chiralität.svg "Spiroverbindung_Chiralität.svg"){width="200"}

*This compound exhibits spiral chirality.*

## Properties of optical isomers

Enantiomers have -- *when present in a symmetric environment* --
identical chemical and physical properties except for their ability to
rotate plane-polarized light by equal amounts but in opposite
directions. A solution of equal parts of an optically-active isomer and
its enantiomer is known as a racemic solution and has a net rotation of
plane-polarized light of zero.

Enantiomers differ in how they interact with different optical isomers
of other compounds. In nature, most biological compounds (such as amino
acids) occur as single enantiomers. As a result, different enantiomers
of a compound may have substantially different biological effects.
Different enantiomers of the same chiral drug can have very different
pharmological effects, mainly because the proteins they bind to are also
chiral.

For example, spearmint leaves and caraway seeds respectively contain
L-carvone and D-carvone -- enantiomers of carvone. These smell different
to most people because our taste receptors also contain chiral molecules
which behave differently in the presence of different enantiomers.

!Limonene enantiomers have different
smells.{width="300"}

D-form Amino acids tend to taste sweet, whereas L-forms are usually
tasteless. This is again due to our chiral taste molecules. The smells
of oranges and lemons are examples of the D and L enantiomers.

Penicillin\'s activity is stereoselective. The antibiotic only works on
peptide links of D-alanine which occur in the cell walls of bacteria --
but not in humans. The antibiotic can kill only the bacteria, and not
us, because we don\'t have these D-amino acids.

The electric and magnetic fields of polarized light oscillate in a
geometric plane. An axis normal to this plane gives the direction of
energy propagation. Optically active isomers rotate the plane that the
fields oscillate in. The polarized light is actually rotated in a
racemic mixture as well, but it is rotated to the left by one of the two
enantiomers, and to the right by the other, which cancel out to zero net
rotation.

## Chirality in biology

Many biologically-active molecules are chiral, including the
naturally-occurring amino acids (the building blocks of proteins), and
sugars. Interestingly, in biological systems most of these compounds are
of the same chirality: most amino acids are L and sugars are D. The
origin of this homochirality in biology is the subject of much debate.

!Enantiomers of amino
acids.

Chiral objects have different interactions with the two enantiomers of
other chiral objects. Enzymes, which are chiral, often distinguish
between the two enantiomers of a chiral substrate. Imagine an enzyme as
having a glove-like cavity which binds a substrate. If this glove is
right handed, then one enantiomer will fit inside and be bound while the
other enantiomer will have a poor fit and is unlikely to bind.

## Chirality in inorganic chemistry

![\[Ru(2,2\'-bipyridine)~3~\]^2+^](Delta-ruthenium-tris(bipyridine)-cation-3D-balls.png "[Ru(2,2'-bipyridine)3]2+")

Many coordination compounds are chiral; for example the well-known
\[Ru(2,2\'-bipyridine)~3~\]^2+^ complex in which the three bipyridine
ligands adopt a chiral propeller-like arrangement \[7\]. In this case,
the Ru atom may be regarded as a stereogenic centre, with the complex
having point chirality. The two enantiomers of complexes such as
\[Ru(2,2\'-bipyridine)3\]2+ may be designated as Λ (left-handed twist of
the propeller described by the ligands) and Δ (right-handed twist).
Hexol is a chiral cobalt compound.

## More definitions

-   Any non-racemic chiral substance is called **scalemic**
-   A chiral substance is **enantiopure** or **homochiral** when only
    one of two possible enantiomers is present.
-   A chiral substance is **enantioenriched** or **heterochiral** when
    an excess of one enantiomer is present but not to the exclusion of
    the other.
-   **Enantiomeric excess** or **ee** is a measure for how much of one
    enantiomer is present compared to the other. For example, in a
    sample with 40% ee in R, the remaining 60% is racemic with 30% of R
    and 30% of S, so that the total amount of R is 70%.

## Enantiopure preparations

Several strategies exist for the preparation of enantiopure compounds.
The first method is the separation of a racemic mixture into its
isomers. Louis Pasteur in his pioneering work was able to isolate the
isomers of tartaric acid because they crystallize from solution as
crystals with differing symmetry. A less common and more recently
discovered method is by enantiomer self-disproportionation, which is an
advanced technique involving the separation of a primarily racemic
fraction from a nearly enantiopure fraction via column chromatography.

In a non-symmetric environment (such as a biological environment)
enantiomers may react at different speeds with other substances. This is
the basis for *chiral synthesis*, which preserves a molecule\'s desired
chirality by reacting it with or catalyzing it with chiral molecules
capable of maintaining the product\'s chirality in the desired
conformation (using certain chiral molecules to help it keep its
configuration). Other methods also exist and are used by organic
chemists to synthesize only (or maybe only *mostly*) the desired
enantiomer in a given reaction.

## Enantiopure medications

Advances in industrial chemical processes have allowed pharmaceutical
manufacturers to take drugs that were originally marketed in racemic
form and divide them into individual enantiomers, each of which may have
unique properties. For some drugs, such as zopiclone, only one
enantiomer (eszopiclone) is active; the FDA has allowed such
once-generic drugs to be patented and marketed under another name. In
other cases, such as ibuprofen, both enantiomers produce the same
effects. Steroid receptor sites also show stereoisomer specificity.

Examples of racemic mixtures and enantiomers that have been marketed
include:

-   **Ofloxacin** (Floxin) and **Levofloxacin** (Levaquin)
-   **Bupivacaine** (Marcaine) and **Ropivacaine** (Naropin)
-   **Methylphenidate** (Ritalin) and **Dexmethylphenidate** (Focalin)
-   **Cetirizine** (Zyrtec) and **Levocetirizine** (Xyzal)
-   **Albuterol** (Ventolin) and **Levalbuterol** (Xopenex)
-   **Omeprazole** (Prilosec) and **Esomeprazole** (Nexium)
-   **Citalopram** (Celexa / Cipramil) and **Escitalopram** (Lexapro /
    Cipralex)
-   **Zopiclone** (Imovane) and **Eszopiclone** (Lunesta)
-   **Modafinil** (Provigil) and **Armodafinil** (Nuvigil) --- sulfur is
    the chiral center in modafinil, instead of carbon.

Many chiral drugs must be made with high enantiomeric purity due to
potential side-effects of the other enantiomer. (The other enantiomer
may also merely be inactive.)

Consider a racemic sample of thalidomide. One enantiomer was thought to
be effective against morning sickness while the other is now known to be
teratogenic. Unfortunately, in this case administering just one of the
enantiomers to a pregnant patient would still be very dangerous as the
two enantiomers are readily interconverted *in vivo*. Thus, if a person
is given either enantiomer, both the D and L isomers will eventually be
present in the patient\'s serum and so chemical processes may not be
used to mitigate its toxicity.

!Thalidomide
enantiomers.{width="300"}

## See also

Optical
activity

------------------------------------------------------------------------

\<\< Haloalkanes \|
Stereochemistry \| Alcohols

[^1]: <http://www.chemguide.co.uk/organicprops/aminoacids/background.html>


# Organic Chemistry/Foundational concepts of organic chemistry

The purpose of this section is to review topics from freshman
chemistry and build the foundation for
further studies in organic chemistry.

1.  History of organic
    chemistry

2.  Atomic
    structure

3.  Electronegativity

4.  Bonding

5.  Electron dot
    structures

6.  Visualization
7.  Resonance

8.  Acids and
    bases

9.  Nomenclature

------------------------------------------------------------------------

![](Go_To_Organic_Chemistry_Contents_.png "Go_To_Organic_Chemistry_Contents_.png")
\| Alkanes \>\>


# Organic Chemistry/Alkanes

![](Go_To_Organic_Chemistry_Contents_.png "Go_To_Organic_Chemistry_Contents_.png")

Alkanes are the simplest organic molecules, consisting solely of
singly-bonded carbon and hydrogen atoms. Alkanes are used as the basis
for naming the majority of organic compounds (their **nomenclature**).
Alkanes have the general formula C~n~H~2n+2~. Although their
reactivities are often rather uninteresting, they provide an excellent
basis for understanding bonding, conformation, and other important
concepts which can be generalized to more \"useful\" molecules.

# Introduction

thumb\|2,2-dimethylpropane or neopentane.\
An example of an alkane Alkanes
are the simplest and the least reactive
hydrocarbon species containing only carbons
and hydrogens. They are commercially very important, for being the
principal constituent of gasoline and lubricating oils and are
extensively employed in organic chemistry; though the role of pure
alkanes (such as hexanes) is delegated mostly to solvents.

The distinguishing feature of an alkane, making it distinct from other
compounds that also exclusively contain carbon and hydrogen, is its lack
of unsaturation. That is to say, it
contains no double or triple bonds, which are highly reactive in organic
chemistry.

Though not totally devoid of reactivity, their lack of reactivity under
most laboratory conditions makes them a relatively uninteresting, though
very important component of organic chemistry. As you will learn about
later, the energy confined within the carbon-carbon bond and the
carbon-hydrogen bond is quite high and their rapid oxidation produces a
large amount of heat, typically in the form of fire.

As said it is important, not considered very important component in the
chemistry.

## Introductory Definitions

**Organic compounds** contain **carbon** and **hydrogen** by definition
and usually other elements (e.g. **nitrogen** and **oxygen**) as well.
(CO~2~ is not an organic compound because it has no hydrogen).

**Hydrocarbons** are organic compounds that contain carbon and hydrogen
only.

**Alkanes** are hydrocarbons or organic compounds made up of only
carbon-carbon single bonds.Hence they are saturated. (as opposed to
double and triple bonds). The simplest alkane is
**methane.**

## Methane

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```
`<small>`{=html} *On WP:*\
Alkanes `</small>`{=html}

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Methane, (CH~4~, one carbon bonded to four hydrogens) is the simplest
organic molecule. It is a gas at standard temperature and pressure
(STP).

  -------------------------------------------------------------------------
  ![](Methane-2D-flat-small.png "Methane-2D-flat-small.png"){width="150"}
  Methane
  -------------------------------------------------------------------------

This is a flattened, two-dimensional representation of methane that you
will see commonly. The true three-dimensional form of methane does not
have any 90 degree angles between bonded hydrogens. The bonds point to
the four corners of a tetrahedron, forming
cos^-1^(-1/3) ≈ 109.5 degree bond angles. !3D rendering of methane, a
regular tetrahedral
shape.

## Ethane

Two carbons singly bonded to each other with six hydrogens is called
**ethane**.

![](Ethane-flat.png "Ethane-flat.png"){width="80"}

Ethane is the second simplest hydrocarbon molecule. It can be thought of
as two methane molecules attached to each other, but with two fewer
hydrogen atoms. Note that, if we were simply to create a new bond
between the carbon centers of two methane molecules, this would violate
the octet rule for the involved atoms.

There are several common methods to draw organic molecules. They are
often used interchangeably, although some methods work better for one
situation or another. It is important to be familiar with the common
methods, as these are the \"languages\" organic chemists can use to
discuss structure with one another.

# Drawing alkanes

When writing out the alkane structures, you can use different levels of
the shorthand depending on the needs at hand in hand. For example,
pentane can be written out. Its formula is C~5~H~12~.

![](Pentane.svg "Pentane.svg"){width="150"},

or CH~3~--CH~2~--CH~2~--CH~2~--CH~3~,

or CH~3~(CH~2~)~3~CH~3~,

or minimized to
![](Pentane-2D-Skeletal.svg "Pentane-2D-Skeletal.svg"){width="100"}

## Line drawing shorthand

Although non-cyclic alkanes are called straight-chain alkanes they are
technically made of linked chains. This is reflected in the line-drawing
method. Each ending point and bend in the line represents one carbon
atom and each short line represents one single carbon-carbon bond. Every
carbon is assumed to be surrounded with a maximum number of hydrogen
atoms unless shown otherwise.

  -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  ![](Propane-2D-Skeletal.svg "Propane-2D-Skeletal.svg"){width="60"} ![](Butane_simple.svg "Butane_simple.svg"){width="85"} ![](Pentane-2D-Skeletal.svg "Pentane-2D-Skeletal.svg"){width="100"}
  *Propane, butane, pentane*
  -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Structures drawn without explicitly showing all carbon atoms are often
called \"skeletal\" structures, since they represent the skeleton or the
backbone of the molecule. In organic chemistry, carbon is very
frequently used, so chemists know that there is a carbon atom at the
endpoints of every line that is not specifically labeled.

# Conformations

**Conformers**, also called **conformational isomers**, or **rotational
isomers**,or**rotomers** are arrangements of the same molecule made
transiently different by the rotation in space about one or more single
bonds.

Other types of isomer can only be converted from one form to another by
*breaking* bonds, but conformational isomers can be made simply by
*rotating* bonds.

## Newman projections

Newman projections are drawings used to represent different positions of
parts of molecules relative to each other in space. Remember that single
bonds can rotate in space if not impeded. Newman projections represent
different positions of rotating molecule parts.

+----------------------------------+----------------------------------+
| Conformers interconvert readily, | ![](Spinning                     |
| normally thousands of times a    | newman.png "Spinningnewman.png") |
| second as parts of molecules     |                                  |
| spin.                            |                                  |
+----------------------------------+----------------------------------+
| In the following drawings,       | ```{=html}                       |
| methyl groups are on the front   | <div align=center>               |
| and back ends of the molecule    | ```                              |
| and a circle represents all that | ![](Methylnewman.png "Me         |
| lies between.                    | thylnewman.png")`<small>`{=html} |
|                                  | *Note: This is how methyl groups |
|                                  | are represented in Newman        |
|                                  | projections* `</small>`{=html}   |
|                                  |                                  |
|                                  | ```{=html}                       |
|                                  | </div>                           |
|                                  | ```                              |
+----------------------------------+----------------------------------+

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<center>
```
+----------------------------------+----------------------------------+
| ```{=html}                       | ```{=html}                       |
| <center>                         | <center>                         |
| ```                              | ```                              |
| !                               | ! |
| "Staggerednewmanprojection.png") |                                  |
|                                  | ```{=html}                       |
| ```{=html}                       | </center>                        |
| </center>                        | ```                              |
| ```                              |                                  |
+----------------------------------+----------------------------------+
| ```{=html}                       | ```{=html}                       |
| <center>                         | <center>                         |
| ```                              | ```                              |
| **Staggered conformation**       | **Eclipsed conformation**\       |
|                                  | (front end overlaps the back and |
| ```{=html}                       | also unstable)                   |
| </center>                        |                                  |
| ```                              | ```{=html}                       |
|                                  | </center>                        |
|                                  | ```                              |
+----------------------------------+----------------------------------+

```{=html}
</center>
```
## Conformations and energy

Different conformations have different potential energies. The staggered
conformation is at a lower potential energy than the eclipsed
conformation, and is favored. In ethane, the barrier to rotation is
approximately 25 kJ/mol, indicating that each pair of eclipsed hydrogens
raises the energy by about 8 kJ/mol. This number also applies to other
organic compounds which have hydrogen atoms at similar distances from
each other. At very low temperatures all conformations revert to the
stabler( due to minimized vibration of atoms at it\'s mean position) ,
lower energy staggered conformation.

## Steric effects

Steric effects have to do with size. Two bulky objects run into each
other and invade each others space. If we replace one or more hydrogen
atoms on the above Newman projections with a methyl or other group, the
potential energy goes up especially for the eclipsed conformations.

Lets look at a Newman projection of butane as it rotates
counterclockwise around its axes.

```{=html}
<center>
```
+---------+---------+---------+---------+---------+---------+---------+
| ```     | ![{widt | ```     | "){widt | ```     | "){widt | ```     |
| !       | h="50"} | !       | h="50"} | !       | h="50"} | !       |
| [](Newm |         | [](Newm |         | [](Newm |         | [](Newm |
| anproje |         | anproje |         | anproje |         | anproje |
| ction1. |         | ction2. |         | ction3. |         | ction4. |
| png "Ne |         | png "Ne |         | png "Ne |         | png "Ne |
| wmanpro |         | wmanpro |         | wmanpro |         | wmanpro |
| jection |         | jection |         | jection |         | jection |
| 1.png") |         | 2.png") |         | 3.png") |         | 4.png") |
|         |         |         |         |         |         |         |
| ```     |         | ```     |         | ```     |         | ```     |
| {=html} |         | {=html} |         | {=html} |         | {=html} |
| </      |         | </      |         | </      |         | </      |
| center> |         | center> |         | center> |         | center> |
| ```     |         | ```     |         | ```     |         | ```     |
+---------+---------+---------+---------+---------+---------+---------+
| ```     |         | ```     |         | ```     |         | ```     |
| {=html} |         | {=html} |         | {=html} |         | {=html} |
| <       |         | <       |         | <       |         | <       |
| center> |         | center> |         | center> |         | center> |
| ```     |         | ```     |         | ```     |         | ```     |
| Anti    |         | E       |         | Gauche  |         | E       |
|         |         | clipsed |         |         |         | clipsed |
| ```     |         |         |         | ```     |         |         |
| {=html} |         | ```     |         | {=html} |         | ```     |
| </      |         | {=html} |         | </      |         | {=html} |
| center> |         | </      |         | center> |         | </      |
| ```     |         | center> |         | ```     |         | center> |
|         |         | ```     |         |         |         | ```     |
+---------+---------+---------+---------+---------+---------+---------+

```{=html}
</center>
```
When the larger groups overlap they repel each other more strongly than
do hydrogen, and the potential energy goes up.

## Entropy

Entropy, represented as a **ΔS**, is a mathematical construct that
represents disorder or probability. Natural systems want to find the
lowest energy or organization possible, which translates to the highest
entropy.

*A note about potential energy: If you are rusty on this, remember the
analogy of a big rock pushed to the top of a hill. At the top it has a
maximum of potential energy. When you push it and allow it to roll down
the hill the potential energy stored in it is transformed into kinetic
energy that can be used to generate heat or smash something.*

Notice that statistically, the ethane molecule has twice as many
opportunities to be in the gauche conformation as in the anti
conformation. However, because the Gauche configuration brings the
methyl groups closer together in space, this generates high energy
steric interactions and do not occur without the input of energy. Thus,
the butane molecules shown will almost never be found in such
unfavorable conformations.

# Preparation of Alkanes

### Wurtz reaction

Wurtz reaction is coupling of haloalkanes using sodium metal in solvent
like dry ether

`<B>`{=html} 2R-X + 2Na → R-R + 2Na^+^X^−^`</B>`{=html}

##### Mechanism

The reaction consists of a halogen-metal exchange involving the free
radical species R• (in a similar fashion to the formation of a Grignard
reagent and then the carbon-carbon bond formation in a nucleophilic
substitution reaction.)

One electron from the metal is transferred to the halogen to produce a
metal halide and an alkyl radical.

:   R-X + M → R• + M^+^X^−^

The alkyl radical then accepts an electron from another metal atom to
form an alkyl anion and the metal becomes cationic. This intermediate
has been isolated in a several cases.

:   R• + M → R^−^M^+^

The nucleophilic carbon of the alkyl anion then displaces the halide in
an S~N~2 reaction, forming a new carbon-carbon covalent bond.

:   R^−^M^+^ + R-X → R-R + M^+^X^−^
:   **COREY-HOUSE reactioN**
:   \[Also called as \'coupling of alkyl halides with organo metallic
    compounds\'\]
:   It is a better method than wurtz reaction. An alkyl halides and a
    lithium dialkyl copper are reacted to give a higher hydrocarbon
:   R\'-X + R~2~CuLi\-\-\--\>R-R\' + R-Cu + LiX
:   (R and R\' may be same or different)
:   It

### Clemmensen reduction

Clemmensen reduction is a reduction of ketones (or aldehydes) to alkanes
using zinc amalgam and hydrochloric acid !The Clemmensen
reduction{width="300"}
The Clemmensen reduction is particularly effective at reducing
aryl-alkyl ketones. With aliphatic or cyclic ketones, zinc metal
reduction is much more effective

The substrate must be stable in the strongly acidic conditions of the
Clemmensen reduction. Acid sensitive substrates should be reacted in the
Wolff-Kishner reduction, which utilizes strongly basic conditions; a
further, milder method is the Mozingo reduction. As a result of
Clemmensen Reduction, the carbon of the carbonyl group involved is
converted from sp^2^ hybridisation to sp^3^ hybridisation. The oxygen
atom is lost in the form of one molecule of water.

### Wolff-Kishner reduction

!The Wolff-Kishner
reduction{width="400"}
The Wolff--Kishner reduction is a chemical reaction that fully reduces a
ketone (or aldehyde) to an alkane. Condensation of the carbonyl compound
with hydrazine forms the hydrazone, and treatment with base induces the
reduction of the carbon coupled with oxidation of the hydrazine to
gaseous nitrogen, to yield the corresponding alkane.

##### Mechanism

!The mechanism of Wolff-Kishner
reduction{width="600"}

The mechanism first involves the formation of the hydrazone in a
mechanism that is probably analogous to the formation of an imine.
Successive deprotonations eventually result in the evolution of
nitrogen. The mechanism can be justified by the evolution of nitrogen as
the thermodynamic driving force. This reaction is also used to
distinguish between aldehydes and ketones.

### Mozingo Reduction

A thioketal is first produced by reaction of the ketone with an
appropriate thiol. The product is then hydrogenolyzed to the alkane,
using Raney nickel
![](Mozingo_reaction.svg "Mozingo_reaction.svg"){width="600"}

# Properties of Alkanes

```{=html}
<div class="noprint" style="border:1px solid gold; background:cornsilk; padding: 4px; text-align: center; float: right;">
```
`<small>`{=html} *On WP:*\
Alkane properties `</small>`{=html}

```{=html}
</div>
```
Alkanes are **not very reactive** when compared with other chemical
species. This is because the backbone carbon atoms in alkanes have
attained their octet of electrons through forming four covalent bonds
(the maximum allowed number of bonds under the octet rule; which is why
carbon\'s valence number is 4). These four bonds formed by carbon in
alkanes are sigma bonds, which are more stable than other types of bond
because of the greater overlap of carbon\'s atomic orbitals with
neighboring atoms\' atomic orbitals. To make alkanes react, the input of
additional energy is needed; either through heat or radiation.

Gasoline is a mixture of the alkanes and unlike many chemicals, can be
stored for long periods and transported without problem. It is only when
ignited that it has enough energy to continue reacting. This property
makes it difficult for alkanes to be converted into other types of
organic molecules. (There are only a few ways to do this). Alkanes are
also **less dense than water**, as one can observe, oil, an alkane,
floats on water.

Alkanes are **non-polar solvents**. Since only C and H atoms are
present, alkanes are nonpolar. Alkanes are
immiscible in water but freely miscible in
other non-polar solvents. Alkanes consisting of weak dipole dipole bonds
can not break the strong hydrogen bond between water molecules hence it
is not miscible in water. The same character is also shown by alkenes.
Because alkanes contain only carbon and hydrogen, combustion produces
compounds that contain only carbon, hydrogen, and/or oxygen. Like other
hydrocarbons, combustion under most circumstances produces mainly carbon
dioxide and water. However, alkanes require more heat to combust and do
not release as much heat when they combust as other classes of
hydrocarbons. Therefore, combustion of alkanes produces higher
concentrations of organic compounds containing oxygen, such as aldehydes
and ketones, when combusting at the same temperature as other
hydrocarbons.

The general formula for alkanes is C~N~H~2N+2~; the simplest possible
alkane is therefore methane, CH~4~. The next simplest is ethane,
C~2~H~6~; the series continues indefinitely. Each carbon atom in an
alkane has sp³ hybridization.

Alkanes are also known as paraffins, or collectively as the paraffin
series. These terms are also used for alkanes whose carbon atoms form a
single, unbranched chain. Branched-chain alkanes are called
isoparaffins.

**Methane** through **Butane** are very flammable gases at standard
temperature and pressure (STP). **Pentane** is an extremely flammable
liquid boiling at 36 °C and boiling points and melting points steadily
increase from there; octadecane is the first alkane which is solid at
room temperature. Longer alkanes are waxy solids; candle wax generally
has between C~20~ and C~25~ chains. As chain length increases ultimately
we reach polyethylene, which consists of carbon chains of indefinite
length, which is generally a hard white solid.

## Chemical properties

Alkanes react only very poorly with ionic or other polar substances. The
pKa values of all alkanes are above 50, and so they are practically
inert to acids and bases. This inertness is the source of the term
paraffins (Latin para + affinis, with the meaning here of \"lacking
affinity\"). In crude oil the alkane molecules have remained chemically
unchanged for millions of years.

However redox reactions of alkanes, in particular with oxygen and the
halogens, are possible as the carbon atoms are in a strongly reduced
condition; in the case of methane, the lowest possible oxidation state
for carbon (−4) is reached. Reaction with oxygen leads to combustion
without any smoke; with halogens, substitution. In addition, alkanes
have been shown to interact with, and bind to, certain transition metal
complexes.

Free radicals, molecules with unpaired electrons, play a large role in
most reactions of alkanes, such as cracking and reformation where
long-chain alkanes are converted into shorter-chain alkanes and
straight-chain alkanes into branched-chain isomers.

In highly branched alkanes and cycloalkanes, the bond angles may differ
significantly from the optimal value (109.5°) in order to allow the
different groups sufficient space. This causes a tension in the
molecule, known as steric hinderance, and can substantially increase the
reactivity. The same is preferred for alkenes too.

# Introduction to Nomenclature

Before we can understand reactions in organic chemistry, we must begin
with a basic knowledge of naming the compounds. The
IUPAC nomenclature
is a system on which most organic chemists have agreed to provide
guidelines to allow them to learn from each others\' works.
Nomenclature, in other words, provides a foundation of language for
organic chemistry.

The names of all alkanes end with *-ane*. Whether or not the carbons are
linked together end-to-end in a ring (called *cyclic alkanes* or
*cycloalkanes*) or whether they contain side chains and branches, the
name of every carbon-hydrogen chain that lacks any double bonds or
functional groups will end with the suffix *-ane*.

Alkanes with unbranched carbon chains are simply named by the number of
carbons in the chain. The first four members of the series (in terms of
number of carbon atoms) are named as follows:

1.  CH~4~ = **methane** = one hydrogen-saturated carbon
2.  C~2~H~6~ = **ethane** = two hydrogen-saturated carbons
3.  C~3~H~8~ = **propane** = three hydrogen-saturated carbons
4.  C~4~H~10~ = **butane** = four hydrogen-saturated carbons

Alkanes with five or more carbon atoms are named by adding the suffix
*-ane* to the appropriate numerical multiplier, except the terminal *-a*
is removed from the basic numerical term. Hence, C~5~H~12~ is called
*pentane*, C~6~H~14~ is called *hexane*, C~7~H~16~ is called *heptane*
and so forth.

Straight-chain alkanes are sometimes indicated by the prefix *n-* (for
normal) to distinguish them from branched-chain alkanes having the same
number of carbon atoms. Although this is not strictly necessary, the
usage is still common in cases where there is an important difference in
properties between the straight-chain and branched-chain isomers: e.g.
*n-hexane* is a neurotoxin while its branched-chain isomers are not.

## Number of hydrogens to carbons

This equation describes the relationship between the number of hydrogen
and carbon atoms in alkanes:

:   **H = 2C + 2**

where \"C\" and \"H\" are used to represent the number of carbon and
hydrogen atoms present in one molecule. If C = 2, then H = 6.

Many textbooks put this in the following format:

:   **C~n~H~2n+2~**

where \"C~n~\" and \"H~2n+2~\" represent the number of carbon and
hydrogen atoms present in one molecule. If C~n~ = 3, then H~2n+2~ =
2(3) + 2 = 8. (For this formula look to the \"n\" for the number, the
\"C\" and the \"H\" letters themselves do not change.)

Progressively longer hydrocarbon chains can be made and are named
systematically, depending on the number of carbons in the longest chain.

## Naming carbon chains up to twelve

-   methane (1 carbon)
-   ethane (2 carbons)
-   propane (3 carbons)
-   butane (4 carbons)
-   pentane (5 carbons)
-   hexane (6 carbons)
-   heptane (7 carbons)
-   octane (8 carbons)
-   nonane (9 carbons)
-   decane (10 carbons)
-   undecane (11 carbons)
-   dodecane (12 carbons)

The prefixes of the first three are the contribution of a German
Chemist, August Wilhelm Hoffman, who also suggested the name quartane
for 4 carbons in 1866. However, the but- prefix had already been in use
since the 1820s and the name quartane never caught on. He also
recommended the endings to use the vowels, a, e, i (or y), o, and u, or
-ane, -ene, -ine or -yne, -one, and -une. Again, only the first three
caught on for single, double, and triple bonds and -one was already in
use for ketones. Pent, hex, hept, oct, and dec all come from the ancient
Greek numbers (penta, hex, hepta, octa, deka) and oddly, non, from the
Latin novem. For longer-chained alkanes we use the special IUPAC
multiplying affixes. For example, pentadecane signifies an alkane with
5+10 = 15 carbon atoms. For chains of length 30, 40, 50, and so on the
basic prefix is added to -contane. For example, C~57~H~116~ is named as
heptapentacontane. When the chain contains 20-29 atoms we have an
exception. C~20~H~42~ is known as icosane, and then we have, e.g.
tetracosane (eliding the \"i\" when necessary). For the length 100 we
have \"hecta\" but for 200, 300 \... 900 we have \"dicta\", \"tricta\",
and so on, eliding the \"i\" on \"icta\" when necessary; for 1000 we
have \"kilia\" but for 2000 and so on, \"dilia\", \"trilia\", and so on,
eliding the \"i\" on \"ilia\" when necessary.

We then put all of the prefixes together in reverse order. The alkane
with 9236 carbon atoms is then hexatridinoniliane.

## Isomerism

The atoms in alkanes with more than three carbon atoms can be arranged
in many ways, leading to a large number of potential different
configurations (isomers). So-called \"normal\" alkanes have a linear,
unbranched configuration, but the *n-* isomer of any given alkane is
only one of potentially hundreds or even possibly millions of
configurations for that number of carbon and hydrogen atoms in some sort
of chain arrangement.\
Isomerism is defined as the compound having same moleculer formula the
formula which present the different moleculer formula arrangement are
called as Isomerism.\
e.g.- The molecular formula for butane is C~4~H~10~.

The number of isomers increases rapidly with the number of carbon atoms
in a given alkane molecule; for alkanes with as few as 12 carbon atoms,
there are over three hundred and fifty-five possible forms the molecule
can take!

:   {\| class=\"wikitable\"

\|- ! \# Carbon Atoms !! \# Isomers of Alkane \|- \| 1 \| 1 \|- \| 2 \|
1 \|- \| 3 \| 1 \|- \| 4 \| 2 \|- \| 5 \| 3 \|- \| 6 \| 5 \|- \| 7 \| 9
\|- \| 8 \| 18 \|- \| 9 \| 35 \|- \| 10 \| 75 \|- \| 11 \| 159 \|- \| 12
\| 355 \|}

## Branched chains

Carbon is able to bond in all four directions and easily forms strong
bonds with other carbon atoms. When one carbon is bonded to more than
two other carbons it forms a branch.

  ------------------------------------------------------------------ ------------------------------------------------------------
  !Isobutane{width="200"}   !Neopentane{width="200"}
  ------------------------------------------------------------------ ------------------------------------------------------------

Above you see a carbon bonded to three and four other carbons.

:   Note: a methane group is called a **methyl** group when it is bonded
    to another carbon instead of a fourth hydrogen. --CH~3~

The common system has naming convention for carbon chains as they relate
to branching.

:   **n-alkanes** are linear
:   **iso-alkanes** have one branch R~2~CH---
:   **neo-alkanes** have two branches R~3~C---

*Note: \"R\" in organic chemistry is a placeholder that can represent
any carbon group.*

## Constitutional isomers

One of the most important characteristics of carbon is its ability to
form **several relatively strong bonds** per atom. It is for this reason
that many scientists believe that carbon is the only element that could
be the basis for the many complicated molecules needed to support a
living being.

One carbon atom can have attached to it not just the one or two other
carbons needed to form a single chain but can bond to up to four other
carbons. It is this ability to bond multiply that allows isomerism.

**Isomers** are two molecules with the same molecular formula but
different physical arrangements. **Constitutional isomers** have their
atoms arranged in a different order. A constitutional isomer of butane
has a main chain that is forked at the end and one carbon shorter in its
main chain than butane.

  ------------------------------------- ----------------------------------------------------------------------------------------------
  !Butane   !Isobutane (2-methyl-propane)"){width="100"}
  ------------------------------------- ----------------------------------------------------------------------------------------------

## Naming Alkanes

There are several ways or systems for the **nomenclature**, or naming,
of organic molecules, but just two main ones.

1.  The traditional, non-systematic names. Many of these linger on,
    especially for simpler or more common molecules.
2.  The systematic **IUPAC**
    (*eye-YOU-pack (International Union of Pure And Applied Chemistry)*)
    names.

The IUPAC system is necessary for complicated organic compounds. It
gives a series of unified rules for naming a large compound by
conceptually dividing it up into smaller, more manageable nameable
units.

Many traditional (non-IUPAC) names are still commonly used in industry,
especially for simpler and more common chemicals, as the traditional
names were already entrenched.

## IUPAC naming rules

```{=html}
<div class="noprint" style="border:1px solid gold; background:cornsilk; padding: 4px; text-align: center; float: right;">
```
`<small>`{=html} *On WP:*\
IUPAC naming
`</small>`{=html}

```{=html}
</div>
```
1.  Find the longest carbon chain, identify the end near which the most
    substituents are located, and
    number the carbons sequentially from that end. This will be the
    parent chain.
2.  Consider all other carbon groups as substituents.
3.  Alphabetize the substituents.
4.  Number the substituents according to the carbon to which they are
    attached. If numbering can be done in more than one way, use the
    numbering system that results in the smallest numbers.

Substituents are named like a parent, and replacing the *-ane* ending
with *-yl*.

### Numbering

![](237trimethyloctanelines.png "237trimethyloctanelines.png")

The above molecule is numbered as follows:

![](237trimethyloctane.png "237trimethyloctane.png")

2,3,7-Trimethyloctane

![](267trimethyloctane.png "267trimethyloctane.png")

**Not** 2,6,7-Trimethyloctane. Remember, number so as to give the
smallest numbers to the substituents.

### Alphabetizing

![](3ethyl3methylpentane.png "3ethyl3methylpentane.png")

3-Ethyl-3-methylpentane

*Ethyl* is listed before *methyl* for alphabetizing purposes.

## Branched Substituents

### Naming branched substituents

![](3-(1-methylethyl)-2,4-dimet.png "3-(1-methylethyl)-2,4-dimet.png")

3-(1-methylethyl)-2,4-dimethylpentane

The main chain in the drawing is numbered 1-5. The main part of the
branched substituent, an ethyl group, is numbered 1\' and 2\'. The
methyl substituent off of the ethyl substituent is not numbered in the
drawing.

To name the compound, put the whole branched substituent name in
parentheses and then number and alphabetize as if a simple substituent.

## Common system

Some prefixes from the common system are accepted in the IUPAC system.

For alphabetization purposes, **iso-** and **neo-** are considered part
of the name, and alphabetized. **Sec-** and **tert-** are not considered
an alphabetizable part of the name.

(In the following images, the **R-** represents any carbon structure.)

### Iso-

**Iso-** can be used for substituents that branch at the second-to-last
carbon and end with two methyls. An isobutyl has four carbons total:

![](Isobutyl.png "Isobutyl.png") *Isobutyl*

### Sec-

![](Sec-.png "Sec-.png")

**Sec-** can be used for substituents that branch at the first carbon .

### Neo-

**Neo-** refers to a substituent whose second-to-last carbon of the
chain is trisubstituted (has three methyl groups attached to it). A
neo-pentyl has five carbons total.

![](Neopentyl.png "Neopentyl.png") *Neopentyl*

### Tert-

![](Tert-.png "Tert-.png")

**Tert-** is short for **tertiary** and refers to a substituent whose
first carbon has three other carbon groups attached to it.

# See also

1.  Cycloalkanes

------------------------------------------------------------------------

![](Go_To_Organic_Chemistry_Contents_.png "Go_To_Organic_Chemistry_Contents_.png")
\|\| \<\< Foundational
concepts
\| Alkanes \| Stereochemistry
\>\>

pl:Chemia_organiczna/Alkany


# Organic Chemistry/Stereochemistry

![](Go_To_Organic_Chemistry_Contents_.png "Go_To_Organic_Chemistry_Contents_.png")
Organic Chemistry table of contents \>

# Stereoisomerism

**Stereoisomers** are compounds that have the same connectivity
(constitution) and the same chemical formula, but are isomers because
they differ in the spacial arrangement of the atoms attached to the
stereocenters (chirality centers) throughout the molecule. All
stereoisomers are unique and possess their own physical, chemical and
biological properties (with the exception of meso compounds).

!Two enantiomers of the common psychoactive drug
methamphetamine{width="450"}

To illustrate the importance of stereoisomerism in organic chemistry,
let\'s consider the well-known substance methamphetamine, which has two
enantiomers: (R)-N-methyl-1-phenylpropan-2-amine and
(S)-N-methyl-1-phenylpropan-2-amine. The S enantiomer, commonly called
D-methamphetamine is a controlled substance in most countries worldwide.
It is highly psychoactive, giving users an intense rush of euphoria when
ingested. Use of D-methamphetamine often leads to severe drug dependence
and even death in chronic users. By contrast, the R enantiomer of
methamphetamine, commonly called L-methamphetamine is a non-psychoactive
substance that is legal to possess in most jurisdictions. While not very
psychoactive, L-methamphetamine is an excellent sympathomimetic
vasoconstrictor and can be found in some OTC medications used to relieve
nasal congestion caused by the common cold or allergic sinusitis, often
under the name \"levmetamfetamine\".

\

## Types of Stereoisomers

**Enantiomers** are chiral molecules with non-superimposable mirror
images. When comparing stereoisomers, the enantiomer is always the
isomer without any internal planes of symmetry and that has had each of
its stereogenic centers flipped. In other words, each stereocenter has
gone from an R-configuration to an S-configuration.

!Two enantiomers of
3-chloro-1-ethylcyclohexane{width="225"}\
In this example, the two molecules are enantiomers of one another
because (1) this molecule lacks any internal plane of symmetry and (2)
all of its stereocenters have been flipped. The molecule on the right is
(1S,3S)-3-chloro-1-ethylcyclohexane, while the molecule on the left is
(1R,3R)-3-chloro-1-ethylcyclohexane.

\
**Diastereomers** are also chiral molecules, but differ from enantiomers
in that they are not mirror images of each other due to some **but not
all** of the chirality centers being flipped.

!Two diastereomers of
3-chloro-1-ethylcyclohexane{width="225"}\
The following two molecules are diastereomers of one another because (1)
they are not mirror images, and (2) only one of the stereocenters has
been flipped. The molecule on the left is
(1R,3S)-3-chloro-1-ethylcyclohaxane while the molecule on the right is
(1S,3R)-3-chloro-1-ethylcyclohexane.

\
**Meso compunds** are achiral molecules that contain stereogenic
centers. They are achiral molecules because their mirror images are
superimposable onto each other as they possess an internal plane of
symmetry, meaning that meso compounds are identical molecules with
identical properties.

!The meso compound
(1R\*,2S\*)-1,3-dichlorohexane.-1,3-dichlorohexane."){width="300"}\
A common mistake is thinking that these two molecules are enantiomeric
due to both stereogenic centers having been flipped. However, upon
closer inspection these two molecules are not enantiomeric because by
manipulating this molecule in space it could be superimposed on top of
its mirror image due to it possessing an internal plane of symmetry.

\

------------------------------------------------------------------------

-   Configurations

-   Enantiomers

:   Optical
    activity

:   R-S notational
    system

:   Meso
    compounds

-   Diastereomers

------------------------------------------------------------------------

\<\< Alkanes and cycloalkanes \|
Stereochemistry \|  Haloalkanes


# Organic Chemistry/Cycloalkanes

![](Go_To_Organic_Chemistry_Contents.png "Go_To_Organic_Chemistry_Contents.png")

## Overview

A *cycloalkane* is a regular alkane with a ring or loop. An example is
cyclohexane, which is a ring of 6 carbon atoms, each bonded to 2
hydrogen atoms (C~6~H~12~).

We briefly discussed cycloalkanes in the
alkanes unit of this book, but
in this unit, we\'ll be going into much greater detail about
cycloalkanes as well as cycloalkenes. There are a number of properties
that are unique to these structures and thus they deserve special
attention.

Because of their cyclical nature, cycloalkanes do not have the freedom
of rotation that regular alkanes possess. Stereochemistry plays a very
important role in both the limitations of movements, but also, in some
instances the limitations of reactions that can take place. Often these
limitations are due less to the cyclical nature itself so much as the
lack of freedom of rotation found in such alkanes.

A good example is the restrictions placed on
**E2**
elimination reactions, which we\'ll cover later in the book.

## Drawing Cycloalkanes

!First four simple
cycloalkanes{width="400"}
The above image of the first four simple cycloalkanes shows that each
carbon has 2 hydrogens attached. As you can see, the other 2 bonds are
to adjacent carbons. While the shapes show the basic 2-dimensionaly,
top-down shape, only cyclopropane and cyclobutane have all of their
carbons on a plane. Cylcopentane and cyclohexane, as well as higher
cycloalkanes, all have a 3-dimensional geometry. Cyclohexane is
particularly important and it will be discussed separately.

!Cyclopentane and cyclohexane without atoms
labeled
More often, cycloalkanes, like many other organic structures, are drawn
such that their carbons and hydrogens aren\'t labeled. Cyclopentane and
cyclohexane (see above) are generally drawn as a pentagon and a hexagon,
respectively.

## Naming of Cycloalkanes

Naming of cycloalkanes is similar to alkanes. The smallest cycloalkane
is \"Cyclopropane\", so as you might imagine, this is a ring of 3
carbons. If the number of carbons on the cycloalkane is higher than the
number of carbons on any of its substituents (that is, carbon chains
attached to carbons on the cycloalkane), then the base name is prefixed
with \"cyclo\" and the rest of the name the same as an alkane with the
same number of carbons.

!3-cyclopentylhexane

If, as in the above image, the cycloalkane has fewer carbons than a
carbon chain it\'s connected to, then the longest carbon chain is the
primary and the cyclic portion receives the substituent \"-yl\" ending
and prefixes the primary name. Thus the name 1-ethylbutylcyclopentane,
while technically an accurate description, is not the correct name.

!3-chloro-1,1-dimethylcyclohexane and
2-bromo-3-chloro-1,1-dimethylcyclohexane

Numbering of carbons in substituted cycloalkanes is fairly
straight-forward. If there is only one substituent, it is the 1-carbon.
If there are more than one, then numbering proceeds, clockwise or
counter-clockwise, such that the lowest number after 1 goes to the least
(or a least) substituted carbon. I know that sounds tricky, so we\'ll
look at the example on the left, 3-chloro-1,1-dimethylcyclohexane. the
carbon with the chlorine bond is the least substituted carbon, so it
gets the lowest number after 1 and 1 goes to the nearest carbon with
more than 1 substituent. So, the 1 is applied to the methyls.
Substituents are still placed in alphabetic order, as you can see from
the second example,
2-**b**romo-3-**c**hloro-1,1-di**m**ethylcyclohexane.

## Ring Strain in Cycloalkanes

Carbons with 4 single bonds want the bonds to be at a tetrahedral angle
of 109.5°. In 1885, Adolf von Baeyer proposed that since carbons prefer
this angle, that only rings of 5 and 6 members should be possible.
Baeyer made the mistake of thinking in 2-dimensions and assuming that
all rings are planar. In fact, only cyclopropane and cyclobutane are
flat, resulting in their bond angles of 60° and 90°, respectively. But
for some time, it was believed that these smaller cycloalkanes could not
be created and that ones of more than 6 carbons would have strain
because their angles would be too far in excess of the 109.5° preferred
angle.

!First four simple cycloalkanes in 3-dimensional stick
representation

In the above figure, the first four simple cycloalkanes are again
represented, but this time the 3 dimensional representations are shown.
Notice how cyclopentane has 4 carbons on a plane and the 5th is slightly
off the plane, giving it the shape of an open envelope.

In fact, rings larger than 3 carbons have 3 dimensional shapes that
relieve this bond strain, but it\'s important to note that bond strain
does affect stability. For cyclopropane and cyclobutane, the strain
energy is about 110 kJ/mol. Cyclobutane can enter a \"puckered\"
formation that slightly relieves some torsional strain. Cyclopentane,
which is non-planar can remove some of the strain and has only about 25
kJ/mol of strain. Cyclohexane, because of its chair conformation, can
maintain perfect tetrahedral angles, resulting in no strain. As the ring
size goes up further, angle strain can be avoided at the cost of
introducing some eclipsing strain, and *vice versa*, so that some strain
exists, but the most strained of these, cyclononane, has only about 50
kJ/mol of strain. Ring strain pervades for ring sizes up to 13 members.
After that, there are enough carbons that ring strain is removed
completely.

!Cyclopentane in a 3-dimensional stick
representation

But angle strain isn\'t the only issue involved in ring strain.
Torsional strain is also a factor. When the hydrogen bonds in a ring
eclipse each other, this creates additional strain. In the case of
cyclopropane the hydrogens eclipse each other, greatly adding to the
strain. In fact, this is the cause of most of the strain found in
cyclopentane. Cyclopentane has bond angles very close to 109.5°, but
because 4 of its carbons are planar, there it has torsional strain that
makes it less stable. Looking at the cyclopentane along 2 of the bonds
on the plane, with the non-planar carbon closest to us, in the figure
above, one can see that the bend in the bonds allows the hydrogens to
move slightly out eclipse, reducing the torsional strain somewhat.

Cyclohexane is the lowest membered ring that\'s able to have both 109.5°
bond angles and maintain a staggered conformation, allowing it to be
strain free.

## Conformations of Cyclohexane

There are 3 main conformations of Cyclohexane, the chair, the reverse
chair and the boat. The chair conformations have no angular, torsional
or steric strain. However the boat conformation does have steric strain,
as the hydrogens at the high points on the front and back of the boat
are slightly overlapping.

## Conformations of Mono- and Di-substituted Cyclohexane

**TO DO**

## The Effect of Conformation on Reactions with Cyclohexane

Different conformations of cyclohexane can react differently for certain
reactions. For example, the **E2** elimination
reaction,
which we\'ll cover later in this book, requires that a halogen atom and
a hydrogen atom be on adjacent carbons and coplanar from each other, or
180° (anti). In cyclohexanes with multiple substituents attached to its
carbons, it\'s possible for that the hydrogen and halogen can be on
adjacent carbons, but never be coplanar, thus preventing the reaction
from occurring.

In other reactions, steric interference from adjacent substituents, for
example, a large alkyl group on an adjacent carbon, can prevent a
nucleophile from attacking a desired location. For these, and other
reasons, it\'s important to understand the 3-dimensional geometry and
structure of the reactants as well as the mechanisms of the reactions
that are taking place.

## Large Ring Systems

**TO DO**

## Polycyclic Ring Systems

In some compounds, two or more rings are fused together in the same
molecule. This can occur in both aliphatic and aromatic compounds.

If the two rings meet at a single carbon atom, where that carbon atom is
part of both rings, the compound is called a spiro
compound. The two rings are roughly (but
not exactly) perpendicular to each other because of the usual
tetrahedral bonding of four ligands around carbon.

If the two rings are bridged at two or more carbon atoms, the carbon
atoms form part of a bridge, and the compound is a bicycloalkane (for
two rings) or a polycycloalkane (for three or more rings).

An aromatic bicyclic compound is naphthalene
(C~10~H~8~), which is a pair of benzene rings fused across a
carbon-carbon bond. An aliphatic (non-aromatic) bicyclic compound is
decalin (C~10~H~18~), formally known as
decahydronaphthalene, which is a pair of cyclohexane rings fused across
a C-C bond.

### Naming conventions for bicyclic rings

A bicyclic alkane ring system is composed of three parts:

1.  The longer carbon chain (**a**)
2.  The shorter carbon chain (**b**)
3.  The carbon-carbon bridge (**c**)

Note that $a >= b >= c$ always.

A bicyclic ring is named according to the **total number of carbons** in
the following form:

:   **bicyclo\[a.b.c\]alkane**

In this formula, substitute \[a.b.c\] with the lengths of the carbon
chains, and substitute \"alkane\" with the total number of carbons
(heptane, octane, nonane, etc.).

A famous example of a bicyclic compound is
norbornane (see image below). The formal name
for norbornane is bicyclo\[2.2.1\]heptane. There are two carbons to the
left of the bridge, two carbons to the right of the bridge, and one
carbon on the bridge itself. Therefore, the numbers in the brackets are
\[2.2.1\]. Note that the two carbons at the base of the bridge are not
counted in the brackets.

!Norbornane.{width="300"}

## Introduction to Heterocyclic Compounds

!A sampling of single-ring heterocylic
compounds

Heterocyclic compounds are cyclic compounds that contain atoms other
than carbon in their ring. The figure above shows just a few single-ring
heterocyclic compounds. The ones shown are simply 5 and 6 membered
rings. There are smaller and larger rings and there are also multiple
ring heterocycles.

Heterocyclic compounds play a big role in organic chemistry and because
they have different electron configurations from carbon, they react
differently from carbon rings and differently from each other. Later in
this book we will discuss several specific heterocyclic compounds that
are very commonly used in organic chemistry and we\'ll discuss their
individual characteristics.

For now, it\'s simply important to know what a heterocyclic compound is.

------------------------------------------------------------------------

![](Go_To_Organic_Chemistry_Contents.png "Go_To_Organic_Chemistry_Contents.png")
\|\| \<\< Haloalkanes \|
Alkanes \| Alcohols \>\>


# Organic Chemistry/Alkynes

The triple carbon bonds is formed in alkynes, due to the absence of
hydrogens, thus allowing carbon bonds to become stronger, due to the
nucleus central force which pulls in nearby atoms

\<\< Alkenes \|**Alkynes**\|
Dienes \>\>

![](Ethyne-2D-flat.png "Ethyne-2D-flat.png"){width="250"} **Alkynes**
are hydrocarbons containing carbon-carbon triple bond. They exhibit
neither geometric nor optical isomerism. The simplest alkyne is ethyne
(HCCH), commonly known as acetylene, as shown at right.

# Multiple Bonds Between Carbon Atoms

As you know from studying alkenes, atoms do not always bond with only
one pair of electrons. In alkenes (as well in other organic and
inorganic molecules) pairs of atoms can share between themselves more
than just a single pair of electrons. Alkynes take this sharing a step
further than alkenes, sharing three electron pairs between carbons
instead of just two.

### Two π Bonds

As you should know already, carbon is generally found in a *tetravalent*
state - it forms four covalent bonds with other atoms. As you know from
the section on alkenes, all four bonds are not necessarily to
*different* atoms, because carbon atoms can double-bond to one another.
What this does is create the appearance of only being bound to three
other atoms, but in actuality four bonds exist.

Alkenes are molecules that consist of carbon and hydrogen atoms where
one or more pairs of carbon atoms participate in a double bond, which
consists of one sigma (σ) and one pi (π) bond. Alkynes are also
molecules consisting of carbon and hydrogen atoms, but instead of
forming a double bond with only one sigma (σ) and one pi (π) bond, the
alkyne has at least one pair of carbon atoms who have a σ and **two** π
bonds \-- a *triple bond*.

The carbon-carbon triple bond, then, is a bond in which the carbon atoms
share an *s* and two *p* orbitals to form just one σ and two π bonds
between them. This results in a linear molecule with a bond angle of
about 180ﾟ. Since we know that double bonds are shorter than single
covalent bonds, it would be logical to predict that the triple bond
would be shorter still, and this is, in fact, the case. The triple
bond's length, 1.20Ǎ, is shorter than that of ethane and ethene's 1.54
and 1.34 angstroms, respectively, but the difference between the triple
and double bonds is slightly less than the difference between the single
and double bonds.

The chemistry is very similar to alkenes in that both are formed by
elimination reactions, and the major chemical reactions that alkynes
undergo are addition type reactions.

### Index Of Hydrogen Deficiency

If we compare the general molecular formulas for the Alkane, Alkene, and
Alkyne families as well as the Cycloalkane and cycloalkene families we
see the following relationship:

  Family        Molecular Formula
  ------------- -------------------
  Alkane        C~n~H~2n+2~
  Alkene        C~n~H~2n~
  Cycloalkane   C~n~H~2n~
  Alkyne        C~n~H~2n-2~
  Cycloalkene   C~n~H~2n-2~

We see that for a ring structure or a double bond there is a difference
of two hydrogens compared to the alkane structure with the same number
of carbons. If there is a ring + double bond (cycloalkene) or a triple
bond (alkyne) then the difference is four hydrogens compared to the
alkane with the same number of carbons. We say that the Index of
Hydrogen Deficiency is equal to the number of pairs of Hydrogens that
must be taken away from the alkane to get the same molecular formula of
the compound under investigation. Every π-bond in the molecule increases
the index by one. Any ring structure increases the index by one. Here is
a list of possibilities:

  Per molecule                    Index of Hydrogen Deficiency
  ------------------------------- ------------------------------
  One double bond                 1
  1 ring                          1
  1 double bond and 1 ring        2
  2 double bonds                  2
  1 triple bond                   2
  1 triple bond + 1 double bond   3
  3 double bonds                  3
  2 double bonds + 1 ring         3
  2 triple bonds                  4
  4 double bonds                  4

Just remember that double bonds have 1 π bond, triple bonds have 2 π
bonds and each π bond is an index of 1. We can use the index and the
molecular formula to identify possibilities as to the exact nature of
the molecule. For example, determine the molecular formula and speculate
on what kind of Pi bonding and/or ring structure the molecules would
have if the Index was given to be 3 and it is a 6 carbon hydrocarbon.

Identify the Alkane molecular formula for six carbons. For n = 6 we
would have C~n~H~2n+2~. That would be C~6~H~14~.

Since an index of 3 means that there are 3 pairs(2) of hydrogen atoms
less in the compound compared with the alkane we determined in step 1,
then we would have C~6~H~14-6~ or C~6~H~8~.

In speculating as to what the bonding and structure could be with an
index of 3 that could mean:

-   -   Three double bonds in a non-cyclic structure like hexatriene
    -   Two double bonds in a ring structure like a cyclohexadiene
    -   One triple bond and one double bond in a non-cyclic structure

Clearly the answer cannot be determined from the formula alone, but the
formula will give important clues as to a molecule\'s structure.

# Cycloalkynes

Cycloalkynes are seldom encountered, and are not stable in small rings
due to angle strain. Cyclooctyne has been isolated, but is very
reactive, and will polymerize with itself quickly. Cyclononyne is the
smallest stable cycloalkyne.

Benzyne is another cycloalkyne that has been proposed as an intermediate
for elimination-addition
reactions of benzene.

# Preparation

In order to synthesize alkynes, one generally starts with a vicinal or
geminal dihalide (an alkane with two halogen atoms attached either next
to one another or across from one another). Adding sodium amide
(NaNH~2~) removes the halogens with regiochemistry subject to Zaitsev\'s
Rule, resulting in a carbon-carbon triple bond due to the loss of both
halogens as well as two hydrogen atoms from the starting molecule. This
is called a double dehydrohalogenation.

## Dehydrogenation of an alkane or alkene

`R-R -(H^2)⇒ CH^2=CH^2`

## Dehalogenation of a tetrahaloalkane

Dehalogenation of tetrahalides in the presence of Zn yields Alkynes.
When the vapors of tetrahalides are passed over heated Zinc. However
this reaction is not so much useful for the preparation of Alkynes.
Because tetrahalides required for this reaction are prepared from
alkynes. So, this reaction can be used as a purification for the
alkynes.

## Dehydrohalogenation of a dihaloalkane

## Through Kolbe\'s Electrolysis

The starting compound is a salt already containing a carbon-carbon
double bond. One such compound is maleic acid. Mechanism is similar to
that of formation of ethane using kolbe\'s electrolysis.\

`CH-COONa`\
`||          (sodium maleate) (maleic acid)`\
`CH-COONa`\
\
`At Anode:                  At cathode:`\
`CH-COO`^`-`^`         2Na`^`+`^` + 2e`^`-`^` -> 2Na`\
`||`\
`CH-COO`^`-`^`         2Na + H`~`2`~`O -> 2NaOH + H`~`2`~\
\
`-2e`^`-`^` ->`\
\
`CH-COO*                                    CH*           CH`\
`||         ->    -2CO`~`2`~`              ->     ||     ->     |||`\
`CH-COO*                                    CH*           CH`\
`(free radical formation)`

In this manner, alkyne is obtained at anode, while NaOH is formed at
cathode and hydrogen gas is liberated.

Vicinal dihalides may be converted into alkynes by using extreme
conditions such as sodium amide NaNH2 typically at 150°C or molten/fused
potassium hydroxide KOH typically at 200°C.

## From Calcium carbide

Calcium carbide is the compound CaC~2~, which consists of calcium ions
(Ca^2+^) and acetylide ions, C~2~^2-^. It is synthesized from lime and
coke in the following reaction:

CaO + 3C → CaC~2~ + CO

This reaction is very endothermic and requires a temperature of 2000^o^
C. For this reason it is produced in an electrical arc furnace.

Calcium carbide may formally be considered a derivative of acetylene, an
extremely weak acid (though not as weak as ammonia). The double
deprotonation means that the carbide ion has very high energy. Instant
hydrolysis occurs when water is added to calcium carbide, yielding
acetylene gas.

CaC~2~ + 2H~2~O → Ca(OH)~2~ + C~2~H~2~

## From alkyl or aryl halides

# Properties

## Physical properties

Most alkynes are **less dense than water** (they float on top of water),
but there are a few exceptions.

## Chemical properties

Liquid alkynes are **non-polar solvents**, immiscible with water.
Alkynes are, however, more polar than alkanes or alkenes, as a result of
the electron density near the triple bond.

Alkynes with a low ratio of hydrogen atoms to carbon atoms are **highly
combustible**. Carbon-carbon triple bonds are highly reactive and easily
broken or converted to double or single bonds. Triple bonds store large
amounts of chemical energy and thus are highly exothermic when broken.
The heat released can cause rapid expansion, so care must be taken when
working with alkynes such as acetylene.

One synthetically important property of **terminal alkynes** is the
acidity of their protons. Whereas the protons in alkanes have p*K*~a~\'s
around 60 and alkene protons have p*K*~a~\'s in the mid-40\'s, terminal
alkynes have p*K*~a~\'s of about 25. Substitution of the alkyne can
reduce the p*K*~a~ of the alkyne even further; for example, PhCCH has a
p*K*~a~ around 23, and Me~3~SiCCH has a p*K*~a~ around 19. The acidity
of alkynes allows them easily to be deprotonated by sufficiently strong
bases, such as butyllithium BuLi or the amide ion NH~2~^-^. More acidic
alkynes such as PhCCH can even be deprotonated by alkoxide bases under
the right conditions.

# Reactions

Alkynes can be hydrated into either a ketone or an aldehyde form. A
(Markovnikov) ketone can be created from an alkyne using a solution of
aqueous sulfuric acid (H~2~O/H~2~SO~4~) and HgSO~4~, whereas the
anti-Markovnikov aldehyde product requires different reagents and is a
multi-step process.

## Oxidative Cleavage of Alkynes

## Hydrohalogenation of Alkynes

Alkynes react very quickly and to completion with hydrogen halides.
Addition is anti, and follows the Markovnikov
Rule.

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RCCH + H-Br (1 equiv) \--\> RCBr=CH~2~\
RCCH + H-Br (2 equivs) \--\> RCBr~2~CH~3~

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Adding a halide acid such as HCl or HBr to an alkyne can create a
geminal dihalide via a Markovnikov process, but adding HBr in the
presence of peroxides results in the Anti-Markovnikov alkenyl bromide
product.

## Halogenation of Alkynes

Adding diatomic halogen molecules such as Br~2~ or Cl~2~ results in
1,2-dihaloalkene, or, if the halogen is added in excess, a
1,1,2,2-tetrahaloalkane.

Adding H~2~O along with the diatomic halide results in a halohydrin
addition and an α-halo ketone.

## Combustion

Alkynes burn in air with a sooty, yellow flame, like alkanes. Alkenes
also burn yellow, while alkanes burn with blue flames. Acetylene burns
with large amounts of heat, and is used in oxyacetylene torches for
welding metals together, for example, in the superstructures of
skyscrapers.

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2 C~2~H~2~ + 5 O~2~ \--\> 4 CO~2~ + 2 H~2~O

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## Reduction

Alkynes can be hydrogenated by adding H~2~ with a metallic catalyst,
such as palladium-carbon or platinum or nickel, which results in both of
the alkyne carbons becoming fully saturated. If Lindlar\'s catalyst is
used instead, the alkyne hydrogenates to a Z enantiomer alkene, and if
an alkali metal in an ammonia solution is used for hydrogenating the
alkyne, an E enantiomer alkene is the result.

### Complete Hydrogenation of Alkynes

As mentioned above, alkynes are reduced to alkanes in the presence of an
active metal catalyst, such as Pt, Pd, Rh, or Ni in the presence of heat
and pressure.

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RCCR\' + 2 H~2~ (Pt cat.)\--\> RCH~2~CH~2~R\'

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### Syn-Hydrogenation of an Alkyne

There are two kinds of addition type reactions where a π-bond is broken
and atoms are added to the molecule. If the atoms are added on the same
side of the molecule then the addition is said to be a \"syn\" addition.
If the added atoms are added on opposite sides of the molecule then the
addition is said to be an \"anti\" addition. Hydrogen atoms can be added
to an alkyne on a one mole to one mole ratio to get an alkene where the
hydrogen atoms have been added on the same side of the molecule.
Isotopic identification allows chemists to determine when this
syn-hydrogenation has occurred.

As mentioned above, alkynes can be reduced to *cis-*alkenes by hydrogen
in the presence of Lindlar Pd, i.e. palladium doped with CaSO~4~ or
BaSO~4~.

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RCCR + H~2~ *cis-*RCH=CHR

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### Anti-Hydrogenation of an Alkyne

In regards to the syn-hydrogenation, anti is hydrogenation when one
hydrogen is added from the top of the pi bond and the other is added
from the bottom. *Anti*-hydrogenation of an alkyne can be done via
metallic reduction, using sodium in liquid ammonia.

## Anion formation

Because of the acidity of the protons of terminal alkynes, they are
easily converted into alkynyl anions in high yield by strong bases.

**Examples**

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RCCH + NaNH~2~ -\> RCCNa + NH~3~ C~4~H~9~Li + RCCH -\> C~4~H~10~ +RCCLi

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Alkynes are stronger bases than water, and acetylene (ethyne) is
produced in a science classroom reaction of calcium carbide with water.

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CaC~2~ + 2 H~2~O \--\> Ca(OH)~2~ + C~2~H~2~

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Alkynyl anions are useful in lengthening carbon chains. They react by
nucleophilic substitution with
alkyl halides.

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R-Cl + R\'CCNa \--\> RCCR\' + NaCl

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The product of this reaction can be reduced to an alkane with hydrogen
and a platinum or rhodium catalyst, or an alkene with Lindlar palladium.

# References

-   IIT Chemistry by Dr.O.P.Agrawal and Avinash Agrawal

\<\< Alkenes \| Alkynes \|
Dienes \>\>