A Guidebook to Mechanism in Organic Chemistry

Electrophilic and Nucleophilic Substitution in Aromatic Systems

p. 88) before it reacts with benzene. In the presence of FeCI 3 as

catalyst, however, the major product is Me 3 C-CH 8 -Ph from the

unrearranged ion, indicating that it never became fully-formed in the

complex. Similarly n-propyl bromide in the presence of gallium bro­

mide, GaBr 3 , yields isopropylbenzene as the major product (p. 85),

whereas n-propyl chloride with aluminium chloride yields very largely

n-propylbenzene.

Alkenes can also be used in place of alkyl halides for alkylating

benzene, the presence of an acid being required to generate a car­

bonium ion; BF 3 is then often used as the Lewis acid catalyst:

Me—CH=CH, Me—CH—Me PhCHMe,

BF,

Several of the usual catalysts, especially AICI 3 , also bring about

ready dealkylation: i.e. the reaction is reversible. Thus heating of

p-xylene (X) with hydrogen chloride and AIC1 3 results in the conver­

sion of a major part of it to the thermodynamically more stable

m-xylene (XI). This is normally explained as taking place by alkylation

(the MeCl necessary to start the process being derived by a little

initial dealkylation) followed by dealkylation:

+ MeCl

(X) Me (XI) Me

The presence of hydrogen chloride is, however, essential for the

isomerisation to take place and it has therefore been suggested that

an alkyl group may also migrate directly by a Wagner-Meerwein type

rearrangement (p. 88):

He

Me

-He

Me
Against this is the fact that //ifer-molecular migration of alkyl
groups has been observed in some cases indicating, unlike a Wagner-
Meerwein rearrangement, that the alkyl group actually becomes free.