Regioselectiviteit

Regioselectivity describes the preference of a chemical reaction to occur at one location or position in a molecule over other possible sites. When multiple constitutional isomers could form, regioselectivity determines which regioisomer predominates as the major product. It is a common consideration in reactions involving unsymmetrical substrates, additions to unsymmetrical alkenes, substitutions on aromatic rings, and many catalytic processes.

The outcome is governed by factors that influence the stability of reactive intermediates and transition states. Electronic effects favor sites that lead to more stable cationic, anionic, or radical intermediates, while steric effects favor less hindered positions. Reaction conditions, including solvent, temperature, and the nature of catalysts or reagents, can also shift regioselectivity. In some cases, kinetic control (fastest-formed product) and thermodynamic control (most stable product) compete and determine the major regioisomer.

Classic examples illustrate the concept. In electrophilic addition to alkenes, HBr adds following Markovnikov’s rule, yielding products where the bromine attaches to the more substituted carbon due to a more stable carbocation intermediate; hydroboration-oxidation, in contrast, gives anti-Markovnikov alcohols via a different regioselective pathway. Allylic bromination using N-bromosuccinimide often targets the most stabilized allylic radical. In electrophilic aromatic substitution, directing groups influence regioselectivity: electron-donating substituents favor ortho/para substitution, while electron-withdrawing groups tend to direct meta substitution.

Regioselectivity is central to synthesis planning and product purity. It can often be manipulated by changing reagents, catalysts, solvents, or reaction conditions. It is distinct from stereoselectivity, which governs the formation of stereoisomers, and from chemoselectivity, which concerns selectivity among different functional groups.