Regiochemie

Regiochemistry, or regiochemistry, is the study of regioselectivity in chemical reactions: the preference for forming one constitutional isomer over others when bonds are formed at different positions on a molecule. It addresses where a new bond is made or a functional group is introduced within a given substrate and how substituents, geometry, and reaction conditions steer the outcome.

Key factors shaping regioselectivity include electronic effects, steric hindrance, and the stability of reactive intermediates or transition states. Substituents can stabilize or destabilize carbocations, carbanions, or radical intermediates, directing the reaction to particular positions. Common rules such as Markovnikov’s rule and anti-Markovnikov outcomes illustrate how electronic distribution influences regioselectivity in electrophilic additions to alkenes; hydroboration-oxidation, for example, delivers anti-Markovnikov alcohols, while traditional hydration yields Markovnikov products. Catalysts, solvents, and temperature can further modulate regioselectivity by altering intermediate stability or providing alternative pathways.

In aromatic chemistry, regiochemistry governs electrophilic aromatic substitution. Directing groups influence regioselectivity, guiding substitution to ortho or para positions with activating groups, or to meta positions with deactivating groups. For instance, methyl directs to ortho and para positions in nitration analyses, reflecting a balance of resonance and steric effects.

Regiochemistry is central to synthetic planning, enabling the selective construction of desired isomers. Quantitative measures, such as product ratios and selectivity factors, help compare competing pathways. Computational and mechanistic studies increasingly aid predictions of regioselective outcomes across diverse reaction types.