The Strengths of Acids and Baseseffect is operating and also to direct interaction, both steric and by
hydrogen bonding, with the NH 2 group {cf the case of o-substituted
benzoic acids, p. 47). o-Nitroaniline is such a weak base that its salts
are largely hydrolysed in aqueous solution, while 2,4-dinitroaniline
is insoluble in aqueous acids, and 2,4,6-trinitroaniline resembles
an amide; it is indeed called picramide and readily undergoes hydro
lysis to picric acid.
With substituents such as OH and OMe having unshared electron
pairs, an electron-donating, i.e. base-strengthening, mesomeric effect
can be exerted from the o- and p-, but not from the m-position with
the result that the ^-substituted aniune is a stronger tfkse than the
corresponding m-compound. The m-compound is a weaker base than
aniline itself due to the electron-withdrawing inductive effect exerted
by the oxygen atom in each case. As so often, the effect of the o-sub-
stituent remains somewhat anomalous due to direct interaction with
the NH 2 group by both steric and polar effects. The substituted
anilines have pK 6 values as follows:
Ph-NH, HOCJHINHJ
9-38 o- 9-28
m- 9-83
8-50•* * *-:OMeAn interesting case is provided by 2,4,6-trinitroaniline (VIII) and
2,4,6-trinitrodimethylaniline (IX), the latter being about forty thou
sand times as strong as the former while aniline itself is only about
twice as strong as dimethylaniline. This is due to the fact that the
influence of nitro groups in the substituted dimethylaniline (IX) is
essentially confined to their inductive effects. The dimethylamino
group is sufficiently large to interfere sterically with the very large nitro
groups in both o-positions, and the p orbitals on the nitrogen atoms
of both NMe 2 and N0 2 groups are thus prevented from becoming
parallel to the orbitals of the nuclear carbon atoms. .As a conse
quence, mesomeric shift of unshared electrons from the amino-nitro-
gen atom to the oxygen atom of the nitro groups via the delocalised
orbitals of the nucleus (cf. p. 52) is also prevented, and base-weaken
ing by mesomeric electron-withdrawal does not, therefore, occur.
In trinitro-aniline (VIII), however, the NH 2 group is sufficiently
small for no such limitation to be imposed and hydrogen-bonding
MeOC 6 H 4 NH 2
o- 9-51- m- 9-80
p- 8-71
H 2 N