THE REACTION BETWEEN SYMMETRICAL ALKENES AND BROMINE

This page gives you the facts and a simple, uncluttered mechanism for the electrophilic addition reactions between bromine (and the other halogens) and alkenes like ethene and cyclohexene. If you want the mechanisms explained to you in detail, there is a link at the bottom of the page.

The electrophilic addition of bromine to ethene

The facts

Alkenes react in the cold with pure liquid bromine, or with a solution of bromine in an organic solvent like tetrachloromethane. The double bond breaks, and a bromine atom becomes attached to each carbon. The bromine loses its original red-brown colour to give a colourless liquid. In the case of the reaction with ethene, 1,2-dibromoethane is formed.

This decolourisation of bromine is often used as a test for a carbon-carbon double bond. If an aqueous solution of bromine is used ("bromine water"), you get a mixture of products. The presence of the water complicates the mechanism beyond what is required by current UK A level (or equivalent) syllabuses.

The other halogens, apart from fluorine, behave similarly. (Fluorine reacts explosively with all hydrocarbons - including alkenes - to give carbon and hydrogen fluoride.)

If you are interested in the reaction with, say, chlorine, all you have to do is to replace Br by Cl in all the equations on this page.

Website:
http://www.chemguide.co.uk/mechanisms/eladd/symbr2.html