The Position of SubstitutionNO,(XVII) (XVIIa) (XVII6)NO, 'NO, © NO,
(XVIII) (XVIIIa) (XVIII6)0,N H 0 2 N H ^
(XlXa) "* (XIX«the group already present has a positively charged nitrogen atom
adjacent to the nucleus so it will clearly not function in helping to.
delocalise the positive charge that is introduced on to the nucleus by
nitration. The three possible intermediates can thus only stabilise
themselves by delocalisation of the charge over the nucleus itself.
This will, however, clearly be less effective if substitution takes place
in the o- and /^-positions, for in each case one of the contributing
structures (XV1I6 and XlXa) would have to carry a positive charge
on a carbon atom which is already bonded to a positively charged
nitrogen atom, a far from stable juxtaposition. With the intermediate-
arising from m-substitution (XVIII) there is no such limitation; this
intermediate is thus more stable than those that would be obtained
by o- or /^-substitution and preferential m-substitution thus takes
place. *
It should be remembered, however, that whatever the nature of the
substituent already present what we are actually considering are the