Addition reaction of alkenes with bromine

Alkenes take part in addition reactions, in which two molecules join to make a single product and nothing else is formed. One of the two bonds in the C=C double bond breaks, and a new atom attaches to each carbon atom.

With ethene and bromine, the bromine molecule splits and one bromine atom joins each carbon atom:

CH₂=CH₂ + Br-Br → CH₂Br-CH₂Br
C₂H₄ + Br₂ → C₂H₄Br₂

The product is dibromoethane. The carbon-carbon bond is now a single bond, so the product is saturated. Each carbon atom still makes four bonds: two to hydrogen, one to bromine and one to the other carbon.

Other alkenes react in exactly the same way. Propene, C₃H₆, takes in one Br₂ molecule to give C₃H₆Br₂, with three carbon atoms, six hydrogen atoms and two bromine atoms. In general, alkene + bromine → dibromoalkane.