IUPAC naming conventions

**IUPAC naming conventions** refer to the means by which organic compounds are systematically named by the **I**nternational **U**nion of **P**ure and **A**pplied **C**hemistry.

## Alkanes

Alkanes are the simplest type of organic molecule, consisting of *just* carbons and hydrogens attached by single bonds. The simplest alkane is methane (CH4) and is called such.

### Straight-Chain Alkanes

So called *straight-chain* alkanes, or those in the form H3C(CH2)n - 2CH3 are called [n]ane where [n] is the Greek prefix for the number n, referring to n carbon atoms. So for instance H3C(CH2)3CH3 would be called pentane, deriving from penta- meaning 5, and H3C(CH2)6CH3 would be called octane, deriving from octa- meaning 8. Exception to this system of Greek prefixes include the first four straight chain alkanes - methane (CH4), ethane (H3CCH3), propane (H3CCH2CH3), and butane (H3C(CH2)2CH3)

### Branched Alkanes

Branched alkanes are those where the carbon atoms are *not* connected by a single non-self-intersecting continuous path, and well, *branch* out! To name a branched alkane, first identify the longest chain of carbons. This is called the parent chain, and determines the final part of the name. For example if the parent chain has four carbons, then the molecule's name ends in butane, because butane also has four carbons. If the parent chain has seven carbons, then the molecule's name ends in heptane, because heptane also has seven carbons.

Sometimes a branched alkane can have more than one parent chain by the definition of longest carbon chain. When this happens, go down both chains and analyze the substituents via the Cahn-Ingold prelog rules, choosing the chain with the point of first difference with a higher priority. You may need to go down each chain both ways to figure this out. Despite this, there may still be branched alkanes with multiple valid paths.

Once you have established the parent chain, record the substituents that branch off the chain. A substituent that is identical to a straight chain [n]-ane minus a hydrogen at the end is called an [n]yl. A methane sans hydrogen substituent is called methyl-, ethane sans hydrogen substituent is called ethyl-, propane sans hydrogen substituent is called propyl-, butane sans hydrogen substituent is called butyl-, and so forth. When listing substituents, do so in alphabetical order. Also note that isopropyl, sec-butyl, and tert-butyl count as substituents in their own right (as long as they are not part of the parent chain), but that sec- and tert- are not counted in the alphabetical ordering of substituents, though isopropyl is. Substituents are called by the number of the carbon they lay on in the parent chain followed by a hyphen followed by the substituent name.

Sometimes there is more than one of a single substituent type on the parent chain. If this is the case, list all carbons the substituent is on, using comas to seperate numbers, and putting the Greek prefix for n where n is the number of that type of substituent before the name of the substituent and after the hyphen. If two of the same type of substituent is on the same carbon atom repeat the number twice.

If a substituent is neither of a straight-chain alkane, isopropyl, sec-butyl, or tert-butyl nature, treat this as a *new* branched alkane ending in the -yl suffix, placing this substituent into parenthesis. The same rules for numbering and multiplicity prefixes on the parent chain apply, although now the first carbon of the substituent's parent chain is forced to be that which branches off of the molecule's parent chain.

### Cycloalkanes

An n-carbon ring where all the carbons are bonded to each other via single bonds is called cycloSC[n] where SC[n] stands for the name of the n-carbon straight-chain alkane. When there is only one substituent, the position need not be denoted. When a cyclo[n]ane is connected to a carbon chain with the same or more carbon atoms, the non-cyclic portion is treated as the parent chain, so that 1-cyclohexylhexane is preferred over hexylcyclohexane.

For two fused cyclo[n]anes one first identifies the largest cyclic [n]ane, then the second largest. Not counting the atoms in the carbon bridge between the two cyclo[n]anes, write the number of carbons in the first cycle (F) followed by the number of carbons in the second cycle (S) followed by the number of carbons in the bridge (B) *excluding* those that are joined to the cycles. Together this is denoted as bicyclo[F.S.B]-. Carbon atoms are labeled beginning with a bridgehead atom, continuing along the larger ring, then on to the second largest ring and finally onto the carbons *on* the bridge. From here the same rules for substituents and alphabetical ordering applies.

## Alkenes and Alkynes

### Straight-Chain Alkenes and Alkynes

Any [n]-ane changed to an [n]-ene retains its carbon structure but will now have a double C=C bond between the first and second carbons. So too, any [n]-ane changed to an [n]-yne retains its carbon structure but will now have a *triple* C≡C bond between the first and second carbons. To denote two or more double or triple bonds in a straight-chain alkene or alkyne, treat the double and triple bonds similar to substituents and write out the first atom they appear on in order on the chain. Double and triple bonds have priority over all other bonds and substituents. Greek infixes for multiplicy of the double and/or triple bonds are used, placed after the prefix for the number of carbons and before the -ene/-yne.

### Cyclic Alkenes and Alkynes

A cyclic [n]-ane amended with the -ene or -yne suffix is identical to the -ane form but with a single double or triple bond instead. The position does not need to be labeled unless there are more -enes or -ynes. Again these have priority over substituents, regardless of what they are.