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Expt 10: Friedel-Crafts Alkylation of p-Xylene
INTRODUCTION
The Friedel-Crafts alkylation reaction is one of the most useful methods for adding
alkyl substituents to an aromatic ring. Mechanistically, this transformation is an
electrophilic aromatic substitution reaction, with the electron-rich aromatic ring serving
as the nucleophile. This electrophile is typically generated
in
situ from an alkyl halide
and a strong Lewis acid such as aluminum chloride (AlCl3).
However, the true nature of the electrophile depends on the structure of the alkyl
halide substrate. For secondary and tertiary alkyl halides the electrophilic species is a
free carbocation. Since secondary and tertiary carbocations are stabilized by
hyperconjugation and inductive effects, the activation energy barrier for their
formation is relatively low.
In contrast, primary and methyl halides are believed to react
via
a polarized complex
since primary and methyl carbocations are typically too high in energy to be viable
intermediates:
One complication that often arises in the Friedel-Crafts alkylation reaction is
rearrangement of the electrophilic species to from a lower energy (i.e. more stable)
intermediate. For example, in CHEM 331, you saw how a secondary carbocation could
rearrange through hydride or methide shifts to form tertiary carbocations. A similar
type of rearrangement can occur when certain primary alkyl halides are used in the
Friedel-Crafts reaction, even though the reaction never proceeds through a free
primary carbocation intermediate. In this case, the hydride shifts occur in the polarized
complex, as shown below for 1-chloropropane:
polarized complex
Cl AlCl3Cl AlCl3
+ AlCl4
tertiary
carbocation
AlCl3H3CCl AlCl3
H3CCl
polarized complex
polarized complex
Cl AlCl3Cl AlCl3
+ AlCl4
secondary
carbocation
HH
