Electrophilic Aromatic Substitution
CH 3 Cl + AlCl 3 →CH 3 δ+Cl+Al−Cl 3
(The carbon atom has a slight excess of positive charge, as the electronegative chlorine atom
draws electron density towards itself. The chlorine atom has a positive charge, as it has
formed a sub-ordinate bond with the aluminium atom. In effect, the Cl atom has lost an
electron, while the Al atom has gained an electron. Therefore, the Al atom has a negative
charge.)
Mechanism of alkylation
The polarized, electrophilic molecule then seeks to saturate its electron deficiency and forms
aπ-complex with the aromatic compound that is rich inπ-electrons. Formation aπ-complex
does not lead to loss of aromaticity. The aromaticity is lost however in theσ-complex that
is the next stage of reaction. The positive charge in theσ-complex is evenly distributed
across the benzene ring.
C 6 H 6 + CH 3 +→C 6 H 6 +Br→C 6 H 5 Br + H+
Theσ-complex C 6 H 6 +Br can be separated (it is stable at low temperatures), while the
π-complex can not.
Restrictions
- Deactivating functional groups, such as nitro (-NO 2 ), usually prevent the reaction from
occurring at any appreciable rate, so it is possible to use solvents such as nitrobenzene
for Friedel-Crafts alkylation. - Primary and secondary carbocations are much less stable than tertiary cations, so rear-
rangement typically occurs when one attempts to introduce primary and secondary alkyl
groups onto the ring. Hence, Friedel-Crafts alkylation using n-butyl chloride generates
the n-butylium cation, which rearranges to the t-butyl cation, which is far more sta-
ble, and the product is exclusively the t-butyl derivative. This may, in some cases, be
circumvented through use of a weaker Lewis acid. - The Friedel-Crafts reaction can not be used to alkylate compounds which are sensitive
to acids, including many heterocycles. - Another factor that restricts the use of Friedel-Crafts alkylation is polyalkylation. Since
alkyl groups have an activating influence, substituted aromatic compounds alkylate more
easily than the original compounds, so that the attempted methylation of benzene to give
toluene often gives significant amounts of xylene and mesitylene. The usual workaround
is to acylate first (see the following sections) and then reduce the carbonyl group to an
alkyl group.
83.3.5 Friedel-Crafts acylation
Figure 176 Friedel-Crafts acylation of benzene by acetyl chloride