Friedel-Crafts Reaction
The main drawback in the preparative use of the Friedel-Crafts
reaction is polyalkylation, however (p. 118).
(ii) Acylation
Acid chlorides or anhydrides in the presence of Lewis acids yield
e
an acylium ion, R-C=0, which acts as the effective electrophile to
form a ketone (XII):
RCOCI+AlCl 3 -> A1C1 4 ©
RCOOCOR + A1C1. RCOOAICI.©
+RC=O
|c.H.
H
-H8
Ph—C—R Ph
II
O
(XII)
The ketone, once formed, complexes with aluminium chloride
C—R
II
O
Ph
C—O—A1CK
removing it from the sphere of reaction. Thus rather more than one
equivalent of the catalyst must be employed, unlike alkylation where
only small amounts are necessary. There is however some evidence
that such AIC1 3 complexing of the ketone is an essential rather than
merely a nuisance feature of the reaction as otherwise the ketone forms
a complex with the acylium ion
Ph O
C—O—CR
and thus prevents the latter from attacking its proper substrate, in
this case C 6 H 6.
Ill