isomerisation

Isomerisation is a chemical process in which a substance is converted into another isomer with the same molecular formula but a different arrangement of atoms or different spatial orientation. Isomerisation can be reversible and is influenced by factors such as temperature, light, and catalysts. It encompasses several related phenomena, spanning structural rearrangements, stereochemical changes, and photochemical transformations.

Isomerisation is commonly categorized into structural and stereochemical forms. Structural isomerisation, or skeletal isomerisation, involves changes in connectivity or the rearrangement of the carbon skeleton, such as tautomerisation in which a proton shifts and a double bond relocates (for example keto–enol tautomerism). Stereochemical isomerisation preserves connectivity but alters spatial arrangement. This includes geometric or cis–trans isomerism around double bonds and conformational changes from rotation about single bonds, such as ring flips in cyclohexane. Photoisomerisation is a form that is driven by light and can convert one stereochemical form into another, as in azobenzene switching between cis and trans forms.

Industrial and biological relevance is broad. In petroleum refining, isomerisation converts straight-chain alkanes to branched isomers to improve octane ratings. In biology, vision relies on photoisomerisation of retinal from 11-cis to all-trans forms. In materials science, isomerisation underpins the function of molecular switches and responsive polymers, where light or heat toggles between isomers to change physical properties. The mechanisms of isomerisation range from acid- or base-catalyzed rearrangements to photochemical and catalytic processes, often involving relatively small energy barriers that enable interconversion under practical conditions.