Electrophilic Addition Reactions Between Unsymmetrical Alkenes and Bromine
Important! This page assumes that you have already read the page on the addition of bromine to symmetrical alkenes. If you haven't, you must read it before you go on. It contains important advice that you will need to make best use of this page.
This page gives you the facts and a simple, uncluttered mechanism for the electrophilic addition reactions between bromine and alkenes like propene. If you want the mechanism explained to you in detail, there is a link at the bottom of the page.
An unsymmetrical alkene is one like propene in which the groups or atoms attached to either end of the carbon-carbon double bond are different.
For example, in propene there are a hydrogen and a methyl group at one end, but two hydrogen atoms at the other end of the double bond. But-1-ene is another unsymmetrical alkene.
The Electrophilic Addition of Bromine to Propene
In common with all other alkenes, propene reacts 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 propene, 1,2-dibromopropane is formed.
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.
The Mechanism For the Reaction Between Propene and Bromine
The reaction is an example of electrophilic addition.
Warning! Just as with symmetrical alkenes, there are two versions of the propene / bromine mechanism in common use, and you must know which your examiners will accept.
How you can find out which one your examiners expect is explained on the page on the addition of bromine to symmetrical alkenes.
Bromine as an electrophile
The bromine is a very "polarisable" molecule and the approaching π bond in the propene induces a dipole in the bromine molecule. If you draw this mechanism in an exam, write the words "induced dipole" next to the bromine molecule.
The simplified version of the mechanism
Note: Use this version unless your examiners insist on the more accurate one.
The more accurate version of the mechanism
Note: Don't learn this unless your examiners insist on it. There's no point in making life difficult for yourself.
In the first stage of the reaction, one of the bromine atoms becomes attached to both carbon atoms, with the positive charge being found on the bromine atom. A bromonium ion is formed.
The bromonium ion is then attacked from the back by a bromide ion formed in a nearby reaction.