A Guidebook to Mechanism in Organic Chemistry

Electrophilic Substitution of Other Aromatic Species

(XXII)

That is o- and p- to the activating substituent, Me, but not m- to the

deactivating substituent, N0 2. This is borne out in practice, i.e. where

an ojp- and a m-directive substituent are in competition the latter

can often be looked upon as merely occupying a position in the

nucleus; though any possible steric effects it may exert must also be

taken into account in deciding which positions, out of several alterna­

tives, are likely to be most readily attacked. With two suitably

situated o/p-directive substituents, however, actual competition

does take place. It is not always possible accurately to forecast the

outcome, but normally those groups that exert their effects via un­

shared electron pairs are more potent than those operating via induc­

tive or hyperconjugative effects, possibly due to tfc»added electromeric

effect (p. 123) exerted on approach of the electnaphile. Thus njtprtion

of acet-p-toluidide (XXIII) leads to

Me' Me Me

jj^jY ©NO.

QNH-CO-Me

' (XXIII)

cyirtually no attack at all taking place o- to Me.

ELECTROPHILIC SUBSTITUTIONa0F OTHER AROMATIC SPECIES

With naphthalene, electrophilic substitution, e.g. nitration, takes

place preferentially at the a- rather than the jS-position. This can be

accounted for by the fact that more effective stabilisation by delocal­

isation can take place in the .metastable intermediate or transition

in m-nitrotoluene (XXII), we should expect nitration to take place

at the positions indicated by arrows:

Me