Resonance involved in benzene ring makes the delocalized electrons span effectively over carbon atoms in the benzene ring. It partially stabilizes the arenium ion too. Partial stability of arenium ion makes benzene highly prone to electrophilic substitution reactions. An electrophilic substitution reaction of benzene is one where an electrophile substitutes the hydrogen atom of benzene. As aromaticity of benzene ring is not disturbed in the reaction, these reactions are highly spontaneous in nature. Friedel Crafts acylation and alkylation reactions are few examples of electrophilic substitution reactions of benzene. In these reactions, a hydrogen atom of the benzene ring is replaced with either an acyl group or an alkyl group.
Friedel Crafts alkylation reaction
When benzene is treated with an alkyl halide in the presence of a Lewis acid such as anhydrous aluminium chloride, alkyl benzene is formed. This reaction is popularly known as Friedel Crafts alkylation reaction.
Friedel Crafts acylation reaction
When benzene is treated with an acyl halide in the presence of Lewis acids such as anhydrous aluminium chloride, acyl benzene is formed. This reaction is popularly known as Friedel Crafts acylation reaction.
The mechanism for Friedel Crafts alkylation and acylation reactions of benzene
Friedel Crafts alkylation and acylation reactions involve a three-step mechanism:
Generation of electrophile:
Due to the presence of a Lewis acid, generation of electrophile takes place. As Lewis acid accepts the electron pair from the attacking reagent.
Formation of arenium ion:
The electrophiles generated attacks on the benzene ring to form positively charged cyclohexadienyl cation better called arenium ion containing one sp3 hybridized carbon atom. The positive charge is effectively distributed over three carbon atoms due to resonance which makes it partially stable.
As delocalization of electrons stops at \(sp^3\) hybridized carbon atom, the arenium ion is not aromatic in nature.
Removal of positive charge from the carbocation intermediate:
The arenium ion finally loses its proton from sp3 hybridized carbon to a Lewis base restoring the aromaticity.
A general mechanism for Friedel Craft’s acylation is depicted below:
For a detailed discussion on Friedel Crafts acylation and alkylation reactions of a benzene ring and its mechanism, download Byju’s-the learning app.