Aromatic Compounds
6.0 Friedel-Crafts Alkylation
6.0 Friedel-Crafts Alkylation
Alkyl halides react with benzene in presence of aluminium chloride to yield alkyl benzenes. Alkylation of benzene with alkyl halides in the presence of aluminium chloride was discovered by Charles Friedel and James. M. Crafts in 1877.
A general equation for a Friedel - Crafts alkylation reaction is the following.
Alkyl halides by themselves are insufficiently electrophilic to react with benzene. serves as a Lewis acid catalyst to enhance the electrophilicity of the alkylating agent. The mechanism for the reaction (shown in the following steps, with isopropyl chloride as $R–X$) starts with the formation of carbocation (step I). The carbocation then acts as an electrophile (Step–II) and attacks the benzene ring to form an arenium ion. The arenium ion (Step III) then loses a proton to generate isopropyl benzene. Some times carbocation rearrange to a more stable carbocation.
Mechanism
Step I
Step II
Step III
with $R–X$ is a primary halide, a simple carbocation probably does not form. Instead, the $AlC{l_3}$ forms a complex with the alkyl halide and this complex acts as the electrophile. In the complex the carbon halogen bond is nearly broken and one in which the carbon atom has a considerable $+ve$ charge.
These complexes are so carbocation like that they also undergo typical carbocation rearrangements.
Friedel-Crafts alkylations are not restricted to the use of alkyl halides and $AlC{l_3}$. Many other pairs of reagents that form carbocations (or carbocation like species) may be used as well. These possibilities include the use of a mixture of an alkene and an acid.
A mixture of an alcohol and an acid may also be used.
