Chlorine and bromine add to alkenes to give 1,2-dihaloalkanes. For example, 1,2-dichloroethane is synthesized by adding chlorine to ethene.

F2 and I2 are not used as reactants in this reaction. Fluor reacts explosively with alkenes and the addition of I 2 is thermodynamically unfavorable.

The mechanism of this reaction involves the formation of the bromonium ion in a first stage. In the second step, Br - acts as a nucleophile, opening the cycle of the bromonium ion to form a 1,2-dibromoalkane.

Stage 1. Double bond attack on the previously polarized bromine molecule.

halogen addition 02

Stage 2. Opening of the halonium (bromonium) ion by bromide attack on the most substituted carbon.

halogen addition 03

[1] trans-But-2-ene

[2] Bromonium ion.

[3] Dihaloalkane "ANTI"