The Friedel-Crafts alkylation synthesizes alkylated products, such as alkylbenzenes, via the reaction of alkyl halides or alkenes with aromatic hydrocarbons. The reaction removes a hydrogen atom on the aromatic ring and replaces it with an electrophile. Thus, it occurs via electrophilic aromatic substitution and is catalyzed using strong Lewis acids, such as AlCl3 or FeCl3. Depending on the number of substituents on the alkyl halide (i.e., primary, secondary, or tertiary alkyl halide), the positively charged alkyl species interacts with the aromatic ring as a simple carbocation or as a carbocation complex of the Lewis acid.
The mechanism of the Friedel-Crafts alkylation occurs in several steps. Initially, the alkyl halide and Lewis acid react to form the carbocation. The carbocation then attacks the aromatic ring breaking one of the ring double bonds, resulting in the formation of a non-aromatic intermediate. Deprotonation of this intermediate then occurs, the ring regains aromaticity and the removed proton forms an acid that restores the Lewis acid catalyst.