Alkanes

Alkanes

The first things you'll learn in organic chemistry are hydrocarbons, and alkanes.

## Alkane Uses

* Methane: natural gas, used for heating and cooking

* Ethane: fuel, and is industrially used in the making of ethylene by the process of steam cracking

* Propane: used for heating, cooking, and fuel

* Butane: fuel used for 'backpack stoves'

* Hexane: used in the formulation of glue for shoes, leather products, and roofing, also used to extract cooking oils from seeds

## The Basics

**DEFINE**: Hydrocarbon: A compound containing only carbon and hydrogen

Carbon has four valence electrons. This means it can form bonds to four other atoms. If one or more of the four bonds connects to another carbon atom, each new carbon atom can connect to three other atoms, and so on. The complexity of the molecules increase with each carbon added.

There are three typical ways to represent a hydrocarbon formula, but four total. These are structural, line, condensed, and molecular diagrams.

**EXAMPLES**:

C3H8 can be shown three ways, with a structural diagram, a condensed diagram, and a molecular diagram.

Structural:

Condensed: CH3-CH2-CH3 (or CH3CH2CH3)

Molecular: C3H8

Of the following hydrocarbons, each molecule differs by the number of carbon atoms that are linked together to form a ‘carbon chain.’ Chains of carbon atoms extend in straight line fashion, so the following hydrocarbons are called ’straight-chain’ or ‘unbranched’ hydrocarbons.

* ethane: C2H6, CH3-CH3
* butane: C4H10, CH3CH2CH2CH3
* pentane C5H12, CH3CH2CH2CH2CH3
* hexane: C6H14, CH3CH2CH2CH2CH2CH3
* heptane: C7H16
* octane: C8H18
* nonane: C9H20
* decane: C10H22

As you might guess, from pentane and onwards, the hydrocarbons have names similar to the prefixes that you might remember from earlier in the year. (Mono, di, tri, tetra, pent, hexa, hepta, octa, nona, deca) They have been changed, as follows, (meth, eth, prop, but, pent, hex, hept, oct, non, dec)

The amount of carbon matches with the prefix, while the hydrogen increases by two from the previous hydrocarbon.

**Try writing the condensed structures of heptane, octane, nonane, and decane. Go on, I’ll wait.**

**ANSWERS:**

heptane: CH3CH2CH2CH2CH2CH2CH3.

octane: CH3CH2CH2CH2CH2CH2CH2CH3.

nonane: CH3CH2CH2CH2CH2CH2CH2CH2CH3.

decane: CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3

## Alkanes

**DEFINE**: Alkane: a hydrocarbon wherein all the carbon atoms are connected by single bonds.

**NOTE:** The hydrocarbons end in ‘ane’ because they are alkanes. An alkane is also called a ‘saturated’ hydrocarbon, because each carbon atoms is bonded to the maximum possible number of other atoms. The carbon’s ability to bond with also atoms is ‘saturated.’

**EXERCISES:**

Name the following hydrocarbons. Draw structural or condensed diagrams for them.

CH4, C2H6, C3H8, C4H10

(**ANSWERS**: methane, ethane, propane, butane)

## Alkyl Groups

A hydrocarbon chain can have side branches, which are also hydrocarbon chains.

These attached branches are called **Alkyl Groups.**

**DEFINE**: An Alkyl Group is an alkane that has lost one hydrogen atom. The unused bond on the carbon atom can be connected to another hydrocarbon chain. This only uses alkyl groups formed by taking a hydrogen off the end of a hydrocarbon chain.

**RULE**: An alkyl group is is named by changing the ‘ane’ ending of the original hydrocarbon to ‘yl’

**EXAMPLES**:

menthane -> methyl

ethane -> ethyl

propane -> propyl

butane -> butyl

etc.

When an alkyl group is attached to another hydrocarbon, the resulting molecule is called a **substituted hydrocarbon** or a **branched hydrocarbon.**

**RULE**: The first step in naming a substituted hydrocarbon is to find the **longest continuous chain of carbon atoms.** The longest chain is called the **Parent Hydrocarbon.**

To find the longest carbon chain, look at every branching point. Decide what branches make the longest overall path. Here, we'll show only carbons are shown to make it easier.

**EXAMPLES**:

The longest path is 7 carbons, so this is heptane.

**EXERCISES**:

Name the parent hydrocarbon

**ANSWER**: 7 carbons, heptane

## Substituted Hydrocarbons

To name a substituted hydrocarbon, the basic idea is to *name the parent hydrocarbon chain* and then n*ame the various alkyl groups* which are attached to the parent hydrocarbon.

**RULE**: A substituted hydrocarbon is named by writing the following one after another.

* the carbon number at which the alkyl group is attached
* a dash
* the name of the alkyl group
* the name of the parent hydrocarbon chain, to which the alkyl group is attached.

**NOTE**: The carbon atoms in the parent hydrocarbon are numbered consecutively from the end of the hydrocarbon which gives the lowest possible set of numbers to the attached groups.

**EXAMPLES**:

**EXERCISE**: Draw a diagram for the following: 3-methylheptane

**ANSWER**:

## Bonds and Naming

Since carbons have four bonds, count the bonds between a given carbon atom and its neighbours, and subtract that number from four. The difference equals the number of hydrogen atoms which must be attached to that carbon. The number of bonds to hydrogens plus the number of bonds to other carbons equals four. The required number of hydrogens is then written into the formula.

**EXAMPLE**:

When a carbon is attached to ONE other carbon

4 bonds - 1 bond used = 3 H’s and C-CH3 = CH3-CH3

**RULE**: If more than one different alkyl group is attached to a hydrocarbon, then:

* list the alkyl groups alphabetically
* put a number before each group
* put a dash between each alkyl group and it’s number

**REMEMBER:**Words don't like numbers! Keep them seperated by dashes. Also, the alphabet has priority over numbers in akyl groups. For example, 5-ethyl-3-methyl-6-propyldecane is correct.

**RULE**: if an alkyl group is repeated:

* list each carbon number where the repeated group is attached, separated by commas
* prefix the repeated group with di, tri, tetra, etc. to show many many identical groups are attached

**EXAMPLE:**

**EXERCISE**: Name the molecule

a)

Sketch the molecules:

b) 3-ethyl-2, 3-dimethylhexane

c) 3, 4-diethylhexane

**ANSWERS:**

ARE YOU READY FOR A QUIZ? http://www.sciencegeek.net/APchemistry/APtaters/alkanes.htm