Chemistry, Third edition

17 · ORGANIC CHEMISTRY: HYDROCARBONS

Naming alkanes

The rules for naming alkanes are as follows:

1.Identify the longest carbon chain.

2.Identify the ‘branches’ on the longest carbon chain and name them according to

the number of carbon atoms they contain, together with the ending ‘yl’: methyl

(1), ethyl (2), propyl (3), butyl (4) and so on. Note that these groups are called

alkylgroups and have the general symbol R–. For example, R–H could mean

CH 3 – H, CH 3 CH 2 – H or CH 3 CH 2 CH 2 – H, etc.

3.Number the carbon atoms on the longest carbon chain to describe the positions

of the branches – use the lowest numbers possible.

4.Write the branches in alphabetical order.

5.If there are more than one branch with the same name, use the prefixes di-, tri-,

tetra- etc.

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Example 17.1

Name the compound

CH 3 CH 2 CH 2 CHCH 2 CH 3

|

CH 3

Answer

1.Identify the longest carbon chain:

CH 3 CH 2 CH 2 CHCH 2 CH 3 hexane

|

CH 3

So, the compound is derived from hexane, the alkane with six carbon atoms.

2.Identify the branch:

CH 3 CH 2 CH 2 CHCH 2 CH 3 methylhexane

|

CH 3 

The chain has a branch with one carbon atom – a methyl group.

3.Number the position of the branch:

This can be done as

1 2 3456

CH 3 CH 2 CH 2 CHCH 2 CH 3

|

CH 3

or as

654321

CH 3 CH 2 CH 2 CHCH 2 CH 3 3-methylhexane

|

CH 3

In the last structure, the methyl group is on the carbon atom with the lowest

number. The name of the compound is therefore 3-methylhexaneand not

4-methylhexane.