Electrophilic and Nucleophilic Substitution in Aromatic Systems
Group introduced Cl NO, Br SOsHPercentage o-^39 30 11 %
Percentage p- 55 70 87 100That a steric factor is not the only one at work, however, is seen in
the nitration of fluoro-, chloro-, bromo^ and iodobenzenes where the
percentage of o-isomer obtained increases as we go along the series,
despite the increase in size of the substituent. This is due to the fact
that the electron-withdrawing inductive effect influences the adjacent
o-positions much more powerfully than the ihore distant p-position.
The inductive effecrtfecreases considerably on going from fluoro- to
iodoTJBBzene (the biggest change being seen in going from fluoro- to
chlorobenzene) resulting in easier attack at the o-positions despite the
increasing size of the group already present.
With o/p-directive groups having unshared electrons, e.g. OMe,
the metastable intermediate leading to /--substitution has a contribu
tion from a quasi p-quinohoid structure (XVIc, p. 120), as compared
with the intermediate leading to o-substitution which has a contribu
tion from a quasi o-quinonoid structure (XIVc, p. 120); as with the
corresponding quinones themselves, the former is likely to be more
stable than the latter thus leading to preferential p-substitutioh.
The o/p-ratio is also a good deal influenced by the actual conditions,
e.g. temperature, under which substitution is carried out, arid there'
are a number of anomalies that have not yet been adequately
explained.
COMPETITION BETWEEN SUBSTITUENTS
If two substituents are already present in a benzene nucleus, the posi
tion of entry of a third can, in a number of cases, be forecast with fair
accuracy. Thus if an o/p- and a m-directive substituent are present, as
more distant /"-position is not susceptible. In support of this it is
found that as the initial substituent increases in size from CH 3 -Me 3 C,
the proportion of the o-isomer obtained drops markedly (57 per cent ->
12 per cent) while that of the p- increases (40 per cent-80 per cent).
Increase in size of the attacking agent has the same effect; thus
substitution of chlorobenzene leads to: