Skip to main content
Chemistry LibreTexts

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)