Friedel-Crafts alkylation

The Friedel-Crafts alkylation is a name reaction in organic chemistry. It is used to add alkyl groups to aromatic rings using a Lewis acid like aluminium chloride as a catalyst.

The Friedel-Crafts alkylation starts with an aromatic compound like benzene (C₆H₆) and an alkyl halide like ethyl chloride (CH₃CH₂Cl). A small amount of a Lewis acid, usually aluminium chloride, is added to be a catalyst.

Alkenes can be used in place of alkyl halides by using a Brønsted acid as the catalyst. This version of the reaction is common in the chemical industry.

The Friedel-Crafts alkylation is a type of electrophilic aromatic substitution with two carbocation intermediates.^[1] First, the alkyl halide reacts with the catalyst, making a carbocation and an anionic Lewis adduct:

CH₃CH₂Cl + AlCl₃ → CH₃CH+2 + AlCl−4

If starting from an alkene and Brønsted acid, the carbocation is instead made from protonation of the alkene:

H₂C=CH₂ + H₂SO₄ → CH₃CH+2 + HSO−4

The carbocation is a strong electrophile, so it is attracted to the many electrons in the aromatic ring. The carbocation reacts with the ring, making a new covalent bond between carbon atoms to form a larger arenium intermediate,

CH₃CH+2 + C₆H₆ → C₆H₆(CH₂CH₃)⁺

Finally, the adduct abstracts a hydrogen ion from the larger carbocation, which regenerates the catalyst and releases the hydrogen halide byproduct:

C₆H₆(CH₂CH₃)⁺ + AlCl−4 → C₆H₅(CH₂CH₃) + AlCl₃ + HCl

The net reaction is

C₆H₆ + CH₃CH₂Cl → C₆H₅(CH₂CH₃) + HCl