Regioselektive

Regioselektive, or regioselectivity, describes a chemical reaction that preferentially occurs at one location within a molecule when several possible reactive sites exist. The result is the formation of one major regioisomer, with other positional isomers formed in smaller amounts. Regioselectivity is a form of chemoselectivity focused on the position of bond formation rather than the type of transformation, and it reflects how electronic and steric factors influence the reaction pathway.

In organic synthesis, regioselectivity is particularly important for reactions involving multiple potential sites, such as additions to alkenes or substitutions on aromatic rings. For example, electrophilic addition to alkenes often follows Markovnikov’s rule, placing the electrophile on the more substituted carbon and hydrogen on the less substituted one; radical pathways or specific reagents can invert this outcome, giving anti-Markovnikov products. In electrophilic aromatic substitution, directing groups and ring substituents steer new substituents to ortho, meta, or para positions, producing characteristic regioisomer patterns.

Factors controlling regioselectivity include electronic effects (stabilization of intermediates like carbocations or radical species), steric hindrance near competing sites, and the nature of the reaction mechanism. Catalysts, reagents, solvents, and temperature can be tuned to favor a desired regioisomer. Regioselectivity is a central consideration in synthetic planning, enabling selective formation of target products and influencing catalyst design and reaction development.