Friedel Crafts reactions of acylation and alkylation are organic chemical reactions that were developed by French chemist Charles Friedel and American chemist James Crafts in the year 1877. These reactions offered new ways to attach substituents to aromatic rings.
Delocalized electrons span effectively over carbon atoms in the benzene ring due to the Resonance involved in the benzene ring. The arenium ion is partially stabilized by this too. The benzene is quite prone to an electrophilic substitution reaction due to this partial stability exhibited by the arenium ion. The hydrogen atom of the benzene is now substituted by the electrophile in an electrophilic substitution reaction involving benzene.
An example illustrating both types of the Friedel Crafts reaction of benzene is given below.
Since the aromaticity of the benzene ring isn’t disturbed in the reaction, these types of reactions have a highly spontaneous nature. Friedel Crafts acylation and alkylation reactions can be considered examples of electrophilic substitution reactions of benzene. An acyl or alkyl group replaces a hydrogen atom belonging to the benzene ring in these reactions.
Friedel Crafts alkylation reaction
The treatment of the aromatic ring with an alkyl halide with a Lewis acid (anhydrous Aluminium Chloride, for example) present leads to the formation of alkylbenzene. The reaction described above is commonly known as Friedel Crafts alkylation reaction.
Friedel Crafts acylation reaction
The treatment of the aromatic ring with an acyl halide with a Lewis acid (anhydrous Aluminium chloride, for example) leads to the formation of acyl benzene. The reaction described above is commonly 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 the sp3 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.