Including a branch in the name
For branched molecules, the name has three parts (prefix + stem + suffix)
The prefix indicates the nature of the branch and the position of the branch on the main carbon chain.
The number indicates the carbon in the longest chain to which the branch is attached. For multiple branches of the same size, the two numbers are separated by a comma. The number is deduced for
alkanes by counting from the end carbon of the longest chain closest to the branch.
alkenes and alkynes by giving the multiply-bonded carbons the lowest possible numbers and numbering the carbon bearing the branch accordingly.
The molecule at the right is 2-methylpentane.
Numbers are used to indicate branch positions and the location of the double bond only if required to unambiguously specify the structure. CH3CH=CH2 is propene not prop-1-ene.
The molecule at the right is methylbutane (not 2–methylbutane).
| Number of carbons | stem | Number of carbons | stem |
| 1 | meth | 5 | pent |
| 2 | eth | 6 | hex |
| 3 | prop | 7 | hept |
| 4 | but | 8 | oct |
The prefix indicates the nature of the branch and the position of the branch on the main carbon chain.
The prefix for a saturated hydrocarbon branch is stem (indicating the number of carbons in the branch) combined with -yl ending. di- indicates two branches of the same size.
CH3CH2– ethyl
CH3– methyl
The number indicates the carbon in the longest chain to which the branch is attached. For multiple branches of the same size, the two numbers are separated by a comma. The number is deduced for
alkanes by counting from the end carbon of the longest chain closest to the branch.
alkenes and alkynes by giving the multiply-bonded carbons the lowest possible numbers and numbering the carbon bearing the branch accordingly.
The molecule at the right is 2-methylpentane.
Numbers are used to indicate branch positions and the location of the double bond only if required to unambiguously specify the structure. CH3CH=CH2 is propene not prop-1-ene.
The molecule at the right is methylbutane (not 2–methylbutane).