stereoisomerisaatioon

Stereoisomerisation is the chemical process by which a molecule changes its stereoisomeric form without changing the connectivity of its atoms. Stereoisomers are compounds that have the same sequence of bonded atoms but differ in the three-dimensional arrangement of those atoms. The process can involve interconversion of enantiomers (mirror-image isomers) and/or diastereomers, such as geometric isomers around a double bond (cis/trans) or around cyclic systems (atropisomers).

Interconversion can occur through various mechanisms. Photochemical or thermal energy can promote rotation about a carbon–carbon double bond or inversion at stereogenic centers, often via transient excited states or reactive intermediates. Catalysis (acid, base, or metal catalysts), radical pathways, or rearrangements can also drive stereoisomerisation. Rates depend on temperature, light exposure, solvent, and substituents; some stereoisomers racemize, yielding a mixture of enantiomers and reducing optical purity.

The ability to control stereoisomerisation is important in synthesis and pharmaceuticals, where the biological activity of enantiomers or diastereomers can differ markedly. Analytical methods such as chiral chromatography, polarimetry, and NMR with chiral shift reagents are used to monitor stereoisomerisation and to quantify enantiomeric excess.

Examples include cis–trans isomerisation of alkenes under light or heat and racemization at stereogenic centers under acidic or basic conditions. The term encompasses both simple geometric isomerisation and more complex configurational changes in chiral molecules.