A Guidebook to Mechanism in Organic Chemistry

Electrophilic and Nucleophilic Substitution in Aromatic Systems *

(XXI)

Conversely aniline, normally o/p-directing, becomes in part at least

m-directing in strongly acid solution, dUe to protonation to form

the anilinium cation:

e

H:NH 4

This is due to the fact that there can no longer be any interaction of

the unshared electron pair on nitrogen with the delocalised IT orbitals

of the nucleus, for the former are now involved in bond formation

with the proton that has been taken up and the inductive effect,

drawing electrons away from the nucleus, is now* enormously en­

hanced by the positive charge on nitrogen. The reason that any o-

and /7-nitroanilines are obtained at all under ordinary conditions with

The rate of reaction will remain slower than in benzene itself, however,
due to the overall deactivation of the nucleus by chlorine's inductive
effect in the opposite direction. A very similar situation is encountered
in the addition of unsymmetrical adducts to vinyl halides, e.g.
CHa==CHBr, where the inductive effect controls the rate, but meso-
meric stabilisation of the carbonium ion intermediate governs the
orientation, of addition (p. 142).

(v) Conditions of reaction

The conditions under which an electrophilic substitution reaction
is carried out can modify or even alter completely the directing effect
of a group. Thus phenol is even more powerfully o//>-directing in
alkaline than in neutral or acid solution, for the species undergoing
substitution is then the phenoxide ion (XXI), in which the inductive
effect is now reversed compared with phenol itself and, more
important, a full blown negative charge is available for interaction
with the IT orbital system of the nucleus; the electron density over the
nucleus is thus notably increased: