A Guidebook to Mechanism in Organic Chemistry

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