Chemistry · Grades 10–12

Branched alkanes and locants

Choose a parent chain and explain what a position number means.

Learn the idea

For these simple acyclic alkanes, choose the longest continuous carbon chain.

A methyl group contributes one carbon outside the chosen chain.

Number the chain to give the substituents the appropriate lowest set of locants.

Count total carbons after naming and compare the described connectivity with the starting structure.

The stated parent is a three-carbon propane chain.

Propane.

Three in the parent plus one in the methyl substituent.

For a methyl branch on the same carbon of a butane chain, which locant is preferred: 2 or 3?
1. 3.
2. 2.
3. Either always gives the preferred name.
How many total carbons are in 2,2-dimethylpropane?
A locant such as 2 primarily tells you what?
1. The total number of neutrons.
2. A substituent position on the parent chain.
3. The molecular mass.

2.
Number from the end that gives the branch the lower locant in this simple example.
5
Three parent carbons plus two methyl carbons give five.
A substituent position on the parent chain.
Locants specify positions in a named structure.