Regioisomers

Regioisomers are a type of structural (constitutional) isomer in which two compounds have the same molecular formula and the same kinds of bonds, but differ in the position of a substituent or functional group on a common framework such as a carbon chain or an aromatic ring. The change in location of a group leads to different chemical and physical properties.

As a subset of constitutional isomers, regioisomerism is distinct from stereoisomerism, which involves differences in three-dimensional arrangement rather than where groups are attached. Regioisomeric relationships are often encountered in synthesis when a reaction could place a substituent at several possible positions. Reactions may be regioselective, favoring the formation of one regioisomer over others.

Common examples include ortho-, meta-, and para- isomers on a substituted benzene ring, such as 1,2- vs 1,3- vs 1,4-dichlorobenzene. In alkenes, different positions of the carbon–carbon double bond, as in but-1-ene versus but-2-ene, are also considered regioisomers. The concept also applies to the position of functional groups in more complex structures, where moving a substituent can yield distinct compounds with different reactivity.

Regioisomerism is important in chemistry and pharmacology because different regioisomers can have markedly different properties, including synthesis routes, reactivity, and biological activity.