A Guidebook to Mechanism in Organic Chemistry

5

ELECTROPHILIC AND NUCLEOPHILIC

SUBSTITUTION IN AROMATIC SYSTEMS

REFERENCE has already been made to the structure of benzene and,
in particular, to its delocalised it orbitals (p. 9); the concentration
of negative" charge above and below the plane of the ring-carbon
atoms is thus benzene's most accessible feature:

This concentration of charge might be expected to shield the ring-

carbon atoms from the atta'ck of nucleophilic reagents and, by

contrast, to promote attack by cations, X®^)r electron-deficient

species, i.e. by electrophilic reagents; this is indeed found tojje the

case.

ELECTROPHILIC ATTACK ON BENZENE

(i) n and o complexes

It might be expected that the first phase of reaction would be inter­

action between the approaching electrophile and the delocalised

it orbitals and, in fact, so-called it complexes such as (I) are formed:

Thus toluene forms a 1:1 complex with hydrogen chloride at -78°,

the reaction bSing readily reversible. That no actual bond is formed

between a ring-carbon atom and the proton from HC1 is confirmed

by repeating the reaction with DCI; this also yields a it complex, but