How to Find and Number the Longest Chain in a Branched Alkane

By Arthur Winter

Alkanes are not limited to staying in a line — they can have structures that branch. When naming a branched alkane, your first step is to find and number the longest chain.

After you number the parent chain, if you want to name the branched alkane, you need to determine the names of all the substituents that stick off of the parent chain, and then order the substituents alphabetically in front of the parent chain.

This figure shows an example of a branching alkane. As you can see, the alkane with the formula C4H10 can have two possible chemical structures, or isomers — one alkane that’s a straight chain (butane), and one alkane that’s branched (isobutane).

Find the longest chain in a branched alkane

The first, and potentially the trickiest, step in naming a branched alkane is to find the longest chain of carbon atoms in the molecule. This task can be tricky because, as a reader of English, you’re so used to reading from left to right. Often, however, the longest chain of carbons is not the chain that follows simply from left to right, but one that snakes around the molecule in different directions. Organic professors like to make the parent chain one that curves around the molecule and doesn’t necessarily flow from left to right, so you have to keep on your toes to make sure that you’ve spotted the longest carbon chain.

The wrong and right ways to count the parent chain.
The wrong and right ways to count the parent chain.

The longest carbon chain for the molecule shown here is seven carbons long, so the parent name for this alkane is heptane.

Number the longest chain in a branched alkane

Number the parent chain starting with the end that reaches a substituent first. A chain can always be numbered in two ways. For the molecule in this example, the numbering could start at the top and go down, or it could start at the bottom and go up. The correct way to number the parent chain is to start with the end that reaches the first substituent sooner. A substituent is organic-speak for a fragment that comes off of the parent chain.

The right and wrong ways to number the carbons of the parent chain.
The right and wrong ways to number the carbons of the parent chain.

As you can see here, if you number from the top down, the first substituent comes at carbon number three; if you number from the bottom up, the first substituent comes at carbon number four. The correct numbering in this case, then, starts at the top and goes down.