Antiadditsioon

Antiadditsioon is a chemical term describing a type of addition reaction where the two reacting atoms or groups add to opposite sides of a double or triple bond. This contrasts with syn-addition, where the atoms or groups add to the same side. In anti-addition, the addition occurs in a stereochemically trans fashion across the pi system. For example, the addition of halogens like bromine (Br2) to alkenes typically proceeds via anti-addition, forming vicinal dihalides where the halogen atoms are on opposite sides of the original double bond. This stereoselectivity is often a consequence of reaction mechanisms involving cyclic intermediates, such as halonium ions. Understanding the distinction between syn- and anti-addition is crucial in organic chemistry for predicting the stereochemical outcome of reactions and for designing synthetic strategies. Many important reactions in organic synthesis, including hydroboration-oxidation (after subsequent oxidation) and the addition of hydrogen halides under certain conditions, exhibit anti-addition stereochemistry. The specific conditions of a reaction, including the reagent used and the solvent, can influence whether syn- or anti-addition predominates.