Electrophilic and Nucleophilic Substitution in Aromatic Systems *
clearly OMe, being an electron-donating group, is able to stabilise
the intermediate by assisting in the delocalisation of its charge when
the incoming group has entered the ,o- or p-, (XlVa and XVIa), but
no/them-position, (XV). It should be noted that additional structures
(two in each case), in which the positive charge is delocalised merely
'within the benzene nucleus (cf. nitrobenzene below), have, for con
venience, been omitted as their contribution will be essentially the
same whether the incoming substituent has entered the o-; m- or
/»-position. The effect of stabilising a transition state (cf. related a
complex) is to lower the activation energy of the reaction leading to
its formation, and preferential 0/jp-substitution thus takes place. With
nitrobenzene, however,
As the transition state usually resembles the related metastable inter
mediate or a complex reasonably closely energetically (p. 32), it may
be assumed that it also resembles it in structure. Thus structural
features that stabilise a particular a complex might be expected to
stabilise the related transition state in a similar way. Thus considering
the three possibilities for nitration when the initial substituent is
OMe (i.e. with anisole),