Structural isomers are compounds that share the same molecular formula but have different arrangements of the atoms. There are several subgroups of structural isomerism. In chain isomers the carbon backbone of the compound is rearranged, but still contains the same number of carbon and hydrogen atoms.

There are 4 molecules. The first molecule is butane, C H 3, C H 2, C H 2, C H 3. Butane has an isomer called methyl propane, also known as isobutane. This molecule has the same number of carbon and hydrogen atoms but a different arrangement of them. The structure is similar to propane, C H 3, C H 2, C H 3, but one of the hydrogens bonded to the middle carbon is replaced with a methyl group, C H 3. The third molecule is hexane, C H 3, C H 2, C H 2, C H 2, C H 2, C H 3. Hexane has an isomer called 2,3-dimethyl butane. The structure is similar to butane, C H 3, C H 2, C H 2, C H 3, but two hydrogens, one bonded to the second carbon and one bonded to the third carbon, are replaced with methyl groups.

Figure 1. Examples of pairs of chain isomers. The first two compounds both have the formula C4H10, the last two compounds both have the formula C6H14.

Another type of structural isomers are position isomers, which are based on the position of functional groups in the molecule. This position slightly alters the name of the molecule to indicate where the functional group is located, and it can have a huge influence on the reactions the molecule takes part in. 1-Butene and 2-butene are position isomers as the double bond is positioned differently in otherwise identical compounds.

Branched hydrocarbons are sometimes named based on the prefix they would be assigned if all carbons were in a single chain. E.g. methyl-propane, which has four carbons, often also go by the name isobutane, where iso- denotes that it’s a structural isomer of butane, which can only be methyl-propane.