4.2: Isomerism

Isomers are compounds with the same molecular formula but a different structural arrangement.

Based on the molecular structure, isomers of organic molecules are classified as constitutional isomers or stereoisomers.

Constitutional isomers differ in the connectivity of their constituent atoms. For example, 1-methoxypropane and diethyl ether are constitutional isomers, as they have the same molecular formula, C₄H₁₀O, but their atoms are connected in a different order.

Molecules in the other class of isomers, referred to as stereoisomers, have the same connectivity but differ in the spatial arrangement of their constituent atoms.

For example, cis-2-butene and trans-2-butene are stereoisomers, as they have atoms connected in the same sequence but are in different points in space. While in the cis isomer, the methyl groups are on the same side of the molecule, in the trans isomer, the methyl groups are on the opposite sides of the molecule.

A chiral molecule and its mirror image are stereoisomers of each other. For example, the chiral molecule 2-butanol and its mirror image are stereoisomers, as these two configurations are non-superposable and accordingly exhibit different spatial arrangements of the constituent atoms.

The stereoisomers of an organic molecule are related through a stereocenter: a point in the molecule at which interchanging any two sets of substituents creates the other stereoisomer.

For example, both carbons in cis-2-butene are stereocenters, because an exchange of the methyl and hydrogen groups at either of these atoms creates the other stereoisomer, trans-2-butene.

In 2-butanol, the C-2 atom is the stereocenter, as exchanging any two groups at this atom creates the other stereoisomer of 2-butanol.

Conformations of a molecule, such as the staggered and eclipsed conformations of ethane, also exhibit different spatial arrangements. Nonetheless, such conformations are generally not classified as stereoisomers, as they rapidly interconvert by free rotation about the sigma bond. Stereoisomers, such as the mirror images of 2-butanol, require bond breaking to interconvert.