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Naming Branched-Chain Alkanes

Key Concepts

Name and number the longest carbon chain as for a straight chain alkane.
Identify each of the branches (side-chains).
Use the lowest number combinations for the branches (side-chains).
Name each branch or side-chain (alkyl group) :

methyl CH₃

ethyl C₂H₅

propyl C₃H₇
For more than 1 of the same alkyl group use:

di = 2

tri = 3

tetra = 4
Use commas between numbers, eg, 1,2 or 2,3
Use hyphens between numbers and words, eg, 2-methyl or 2,3-dimethyl
If there is more than 1 type of branch or side-chain, arrange their names alphabetically,
ie, ethyl groups are named before methyl groups which are named before propyl groups

Animated Tutorial

Examples

(1) Name the molecule shown below:

			H \|
	H \|		H-C-H \|		H \|
H-	C	-	C	-	C	-H
	\| H		\| H		\| H

Number the longest carbon chain:
There are 2 possible ways to number the longest chain:
- from left to right (blue numbers)
- from right to left (red numbers)

			H \|
	H \|		H-C-H \|		H \|
H-	⁽¹⁾C₍₃₎	-	⁽²⁾C₍₂₎	-	⁽³⁾C₍₁₎	-H
	\| H		\| H		\| H

Name the longest carbon chain:
The longest carbon chain contains 3 C atoms: prefix = prop
There are only single bonds between all the carbon atoms in the chain: suffix = ane
The name of the longest carbon chain is propane

Identify the branches or side-chains:
The branch or side-chain is shown in pink on the molecule below:

			H \|
	H \|		H-C-H \|		H \|
H-	⁽¹⁾C₍₃₎	-	⁽²⁾C₍₂₎	-	⁽³⁾C₍₁₎	-H
	\| H		\| H		\| H

Name the branch or side-chains:
The branch or side-chain in this molecule has the formula CH₃ so it is a methyl group.

Number the longest carbon chain so that the branch or side-chain is attached to the carbon atom with the lowest number:

			H \|
	H \|		H-C-H \|		H \|
H-	⁽¹⁾C₍₃₎	-	⁽²⁾C₍₂₎	-	⁽³⁾C₍₁₎	-H
	\| H		\| H		\| H

Numbering from left to right (blue numbers) results in the methyl group being attached to carbon number 2.
Numbering from right to left (red numbers) results in the methyl group being attached to carbon number 2.
In this case it doesn't matter whether we number from right to left or from left to right, the methyl group is still attached to carbon atom number 2 in the longest carbon chain.
The beginning of the molecule's name will be 2-methyl

Assemble the name by naming the branches or side-chains before the name of the longest carbon chain:
2-methylpropane
that is, a methyl group is attached to the second carbon atom of a propane molecule.

(2) Write the structural formula for the molecule 2,2-dimethylpropane

Identify the longest carbon chain : propane

Write the structural formula for propane:

	H \|		H \|		H \|
H-	C	-	C	-	C	-H
	\| H		\| H		\| H

Identify the branches or side chains:
There are two methyl groups since the name of the molecule includes the term dimethyl
The structural formula of a methyl group is:

H
|

H- C -H

|

Number the propane chain:

	H \|		H \|		H \|
H-	⁽¹⁾C	-	(2)C	-	(3)C	-H
	\| H		\| H		\| H

Position the branches or side chains on the carbon chain:
From the name 2,2-dimethylpropane we know that there are 2 methyl groups and that they are both attached to the second carbon atom in the propane chain: