A Guidebook to Mechanism in Organic Chemistry

The Position of Substitution

In practice, however, it substitutes ojp- (it also undergoes substitution

on the CH 2 of the vinyl group) owing to the fact that the metastable

intermediates or o- complexes arising from o- and ^-substitution are

stabilised, by delocalisation due to the vinyl group, in a way that the

intermediate arising from m-substitution cannot be (cf. anisole above):

CHjCH, CH=CH 2 CHjCH,

OaN H

o- m- p-

The behaviour of chlorobenzene is interesting for although CI is,

overall, electron-withdrawing and the nucleus is therefore more diffi­

cult to nitrate than is benzene itself (a circumstance normally

associated with m-directive groups) it does nevertheless substitute

ojp-. This is due to the electron pairs on chlorine being able to assist in

the stabilisation by delocalisation of the intermediates for o- and p-

but not for m-substitution (cf. anisole, p. 120):

o- nt- p-

These electron pairs are somewhat more loth than those on oxygen
or nitrogen to interact with the n orbital system of the nucleus
(p. 119), but such interaction is enhanced by a temporary polarisation,
sometimes called the electromeric effect, superimposed on the per­
manent polarisation of the molecule at the close approach of the
attacking electrophile ®N0 2 :