form of the Friedel-Crafts alkylation mechanism is as follows:
an alkyl halide to the Lewis acid aluminum trichloride results
in the formation of an organo-metallic complex. In this complex
the carbon attached to the chlorine has a great deal of positive
charge character (in fact, for practical purposes when dealing
with this reaction, you can think of the partially positive
charge as a carbocation).
electrons in a benzene ring are mildly electrophilic, and
can attack the partially positive carbon to create a non-aromatic
intermediate (note that this intermediate has several resonance
structures, so that it is not as unstable as it might appear).
Elimination of a proton re-establishes the aromaticity of
the ring, and the aluminum trichloride catalyst is regenerated
along with a molecule of hydrochloric acid.
of caution about this reaction: because the aluminum trichloride
generates what can essentially be thought of as a carbocation,
rearrangments can occur to produce a more highly-substituted
Addition of 1-Chloro-2-Methylpropane to benzene with aluminum
trichloride results in the rearranged product, t-butyl benzene,
and not the product that you might initially expect (work
out the mechanism if you cannot see how that product is attained).
of a Friedel-Crafts alkylation: