Structural isomers are subdivided as chain, position, and functional group. Chain isomers occur among the alkanes. For example, there are two chain isomers of butane, C4H10. In n-butane, CH3CH2CH2CH3, the carbon atoms are joined in a so-called straight, or unbranched, chain. In isobutane, CH3CH(CH3)2, the carbon atoms are joined in a branched chain; the isobutane molecule can be visualized as a carbon atom bonded to one hydrogen atom and to three methyl (CH3) groups.
Position isomers occur among substituted alkanes and other compounds. For example, 1-propanol, CH3CH2CH2OH, and 2-propanol, CH3CH(OH)CH3, are position isomers, as are 1-butene, CH2&eq;CHCH2CH3, and 2-butene, CH3CH&eq;CHCH3. Position isomers have similar chemical properties since they differ only in the location of the functional group (e.g., the OH in an alcohol or the double bond in an alkene).
Functional group isomers, on the other hand, have very different chemical properties because differences in their structure give rise to different functional groups. Ethanol and dimethyl ether (see the example, above) are functional group isomers.