Isomers

Isomers are compounds that share the same molecular formula but differ in the arrangement of atoms or in their three-dimensional orientation. These differences lead to distinct physical properties such as boiling point and density, as well as different chemical reactivity.

Structural or constitutional isomers differ in how their atoms are connected. Examples include chain isomers, such as n-butane and isobutane (C4H10); position isomers, such as 1-propanol and 2-propanol (C3H8O); and functional group isomers, such as ethanol (C2H6O) and dimethyl ether (C2H6O).

Stereoisomers have identical connectivity but different spatial arrangement. They include geometric isomers, which arise from restricted rotation around a bond, with cis/trans or E/Z forms such as cis-2-butene and trans-2-butene. Optical isomers, or enantiomers, are non-superimposable mirror images produced by chiral centers, for example the enantiomers of lactic acid. Some stereoisomer pairs are diastereomers, which are not mirror images of each other, such as certain pairs of epimers or cis/trans pairs that are not enantiomeric.

Many compounds may also include special cases such as meso compounds, which have stereocenters but are achiral due to internal symmetry.

Understanding isomerism is important in chemistry and pharmacology because isomers can have very different biological activities and industrial properties, despite sharing the same molecular formula.