# 15.15 The Friedel-Crafts Alkylation of Benzene

This page gives you the facts and a simple, uncluttered mechanism for the electrophilic substitution reaction between benzene and chloromethane in the presence of an aluminium chloride catalyst. Any other chloroalkane would work similarly.

### The electrophilic substitution reaction between benzene and chloromethane

Alkylation means substituting an alkyl group into something - in this case into a benzene ring. A hydrogen on the ring is replaced by a group like methyl or ethyl and so on.

Benzene is treated with a chloroalkane (for example, chloromethane or chloroethane) in the presence of aluminum chloride as a catalyst. On this page, we will look at substituting a methyl group, but any other alkyl group could be used in the same way. Substituting a methyl group gives methylbenzene - once known as toluene.

$C_6H_6 + CH_3Cl \rightarrow C_6H_5CH_3 + HCl$

or better:

The aluminium chloride isn't written into these equations because it is acting as a catalyst. If you wanted to include it, you could write AlCl3 over the top of the arrow.

### The formation of the electrophile

The electrophile is CH3+. It is formed by reaction between the chloromethane and the aluminum chloride catalyst.

$CH_3Cl + AlCl_3 \rightarrow ^+CH_3 + AlCl_4^-$

### The electrophilic substitution mechanism

Stage one

Stage two

The hydrogen is removed by the $$AlCl_4^-$$ ion wh ich was formed at the same time as the $$CH_3^+$$ ele ctrophile . The aluminum chloride catalyst is re-generated in this second stage.

### Contributors

Jim Clark (Chemguide.co.uk)