Alkene Reactions: Dibromide Formation using Br2 and Alkenes

Bromine (Br2) in the presence of alkenes reacts to form dibromides:

Bromine reacts with the alkene molecule to form a vicinal dibromide (1,2-dibromide) where bromines are added to the opposite side of each other in anti addition, giving 2 products that are enantiomers:

Anti Addition

Partial anti Addition

No stereocenters

The reaction mechanism is depicted below:

In the first step, electrons from the alkene bond react with the first bromine to make a bond, and electrons from the first bromine atom react with the other carbon to form another bond, forming a ring structure as an intermediate.

In the second step, the free, negatively charged bromine, attacks the backside of the intermediate structure, causing one bromine to finish with one type of stereochemistry and the other bromine to finish with the opposite type of stereochemistry, resulting in an anti addition..

The reagent used for this reaction is Br2 however, the reagent may be listed as Br2 with CCl4, though this does not change the reaction or the mechanism.