antiAddition

Antiaddition is a class of addition reactions in organic chemistry in which two new substituents add to a carbon–carbon multiple bond from opposite faces of the pi system. This stereochemical outcome is contrasted with syn addition, where both substituents attach to the same face. In most cases, anti addition places the new groups on opposite sides of the former double bond, often yielding trans configurations in the product when the starting alkene is not highly substituted.

Mechanistically, anti addition often proceeds through a bridged intermediate that temporarily blocks one face of the alkene. For example, halogenation of alkenes with bromine or chlorine forms a halonium ion (such as a bromonium ion); a nucleophile then attacks from the opposite face, opening the ring and delivering the two halogen substituents anti to each other. Other anti-addition examples include oxymercuration–demercuration, in which water or alcohol adds to alkenes via a mercurinium ion intermediate. In these cases, the nucleophile attacks from the side opposite the electrophilic metal center, producing anti stereochemistry.

Anti addition is often contrasted with syn addition, in which both new substituents add from the same face. Many common additions, such as hydroboration–oxidation, are predominantly syn, whereas halogenation and oxymercuration examples typically proceed with anti addition. Some reactions can yield mixtures of syn and anti products depending on substrate and conditions, but concerted processes often favor a single stereochemical outcome.

Significance of anti addition lies in its utility for stereochemical analysis and mechanism elucidation. It helps predict and rationalize the relative stereochemistry of products from unsymmetrical alkenes and informs synthetic strategies that require trans substituents.