Stereochemistry of halogen addition to alkenes

Stereochemistry of halogen addition to alkenes

Addition of halogen is usually predominantly trans, i.e., the addition is stereoselective (one stereoisomer predominates over the other), e.g., the addition of bromine to maleic acid gives –dibromosuccinic acid, which can result only from trans addition. To account for this, Roberts et al. (1937) suggested that the first step is the formation of a bridged (or non-classical) carbonium ion, followed by attack by the bromide ion.

The bromide ion attacks from behind and along the bonding C-Br⁺ line, and results in a Walden inversion at this carbon atom. Since the brominium (bromonium) ion is symmetrical, each carbon atom is attacked equally well, and consequently results in the formation of (I) and (II) in equal amounts. (I) and (II) are a pair of enantiomers and hence the product is a racemic modification.

The existence of this bridged ion has been demonstrated by various workers. It should be noted, however, that although addition is usually trans, cis addition may also occur in certain circumstances.