Chapter 18 · Organic Chemistry
18.7Naming Alkanes
12 min · two checks
Predict
What is the IUPAC name of a three-carbon chain with a one-carbon branch on the middle carbon?
The idea
- Name a branched alkane from the longest chain.
- Write a structure from a simple name.
Find the longest continuous carbon chain and name it as the parent alkane. Name branches as alkyl groups: methyl, ethyl, propyl. Number the chain so the first branch gets the lowest possible number. Write the name as number-branch-parent: 2-methylpentane. If there are two methyl groups, use 2,3-dimethyl and a prefix. Prefixes are ignored when you alphabetize substituents for the final order, but they are part of the name. Commas go between numbers, hyphens between numbers and words.
A wrong parent chain is the error that ruins the rest. Count again around corners; the longest chain may not be drawn horizontally. Then check that your name rebuilds the same structure. 2-methylpropane is not called 1-methylpropane, because the longest chain would then be four carbons and the compound would be butane.
Keep these
- Longest chain = parent. Branches = alkyl groups with numbers.
- Use the lowest set of locants.
- Di-, tri- prefixes do not change the fact that you need a number for each branch.
Worked path
Name the alkane whose longest chain is five carbons, with a methyl group on carbon 2.
Observe
Parent: pentane. One substituent: methyl.