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.
\[C_6H_6 + CH_3Cl \rightarrow C_6H_5CH_3 + HCl\]
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
The hydrogen is removed by the \(AlCl_4^-\) ion which was formed at the same time as the \(CH_3^+\) electrophile. The aluminum chloride catalyst is re-generated in this second stage.
Jim Clark (Chemguide.co.uk)