Isomers share a molecular formula but differ in how the atoms are joined. To list them all, change the carbon chain or the position of the functional group, one change at a time.
This lesson is part of SPM Chemistry carbon compounds. You need the naming rules from naming and drawing organic structures.
How do I find every isomer without repeats?
Start with the longest straight chain. Then shorten the main chain by one carbon and place that carbon as a branch on each different position in turn. Stop when no new position exists.
Name every drawing. Two drawings with the same name are one molecule.
Worked example: isomers of C₅H₁₂
- Longest chain, five carbons: pentane.
- Four-carbon chain with a methyl on carbon 2: 2-methylbutane.
- Three-carbon chain with two methyls on carbon 2: 2,2-dimethylpropane.
A methyl on carbon 3 of a four-carbon chain is the same as carbon 2 counted from the other end, so it gives no fourth isomer. There are three in total.
Worked example: isomers with a functional group
C₄H₈ as alkenes has three isomers: but-1-ene, but-2-ene and 2-methylpropene. The first two differ only in the position of the double bond. The third has a branched chain.
For alcohols with formula C₃H₈O, the –OH can be on carbon 1 or carbon 2, giving propan-1-ol and propan-2-ol.
The mistake that loses marks
A student draws butane as a straight line and again as a bent chain, then counts two isomers. Bending the chain does not change which atoms are connected.
The test is the name. Both drawings give butane, so they are the same molecule.
Check yourself
Draw and name the isomers of C₃H₈O that are alcohols. Then state whether propan-1-ol and propan-2-ol are in the same homologous series.
Answer
Propan-1-ol: CH₃CH₂CH₂OH, with –OH on an end carbon. Propan-2-ol: CH₃CH(OH)CH₃, with –OH on the middle carbon.
Both contain –OH, so both are in the alcohol series. They are position isomers of each other. Each carbon in each drawing has four bonds.
What to study next
Go on to writing characteristic organic reactions, where the position of the functional group affects the product. You can also return to identifying homologous series for the family rules.
Record repeated drawing slips in the mistake log and paper-error review. For a teacher to go through isomer drawings with you, see online one-to-one Chemistry tuition.