A Guidebook to Mechanism in Organic Chemistry

Aromatic Bases

Here the aniline molecule is stabilised with respect to the anilinium
cation (VII) and it is, therefore,' energetically w/rprofitable for aniline
to take up a proton; it thus functions as a base with the utmost
reluctance which is reflected in its pK 6 of 9 • 38, compared with that of
cyclohexylamine, 3 • 32. The effect is naturally more pronounced when
further phenyl groups are introduced on nitrogen; thus diphenyl-
amine, Ph 2 NH is an extremely weak base (pKb = 13-2) while triphenyl-
amine, Ph 3 N is, by ordinary standards, not basic at all.
The overriding importance of this mesomeric destabilising of the
anilinium cation (VII) with resuect to the aniline molecule in deter­
mining the t>asic strength of^iniline is confirmed by the relatively
small and irregular effects produced in j>Kb when methyl groups are
introduced on the nitrogen atom or in the ring; for these groups
would not be expected to influence markedly the interaction of the
nitrogen's unshared pair with the delocalised ir orbitals of the benzene
nucleus (cf. the smallfeffect produced" by introducing alkyl groups
into the nucleus of phenol, p. 45). Thus the substituted anilines have
pKb values:
Ph-NHj Ph NHMe Ph-NMe, Me QH 4 NH,
9-38 9-60 , 9-62 o- 9-62
m- 9-33
j 900
The small, base-strengthening inductive effect they usually eiart is
not large enough to modify the destabilisation of the cation to any
significant extent. A group with a more powerful inductive effect, e.g.
NO 2 , has rather more influence. This is intensified when the nitro
group is in the o- or. ^-position for the interaction of the unshared
electron pair of nitrogen with the delocalised it orbitals of the benzene
nucleus is then enhanced and the cation even further destabilised
with respect to the neutral molecule resulting in further weakening of
the base. Thus the nitro-anilines have pKb values:

Ph-NHj 0 2 NC,H 4 NH 2 H 2 N:>

9-38. o- 14-28 • ^

m- 11-55,

P- 13-02

The extra base-weakening effect when the substituent is in the
o-position is due in part to the short distance over which its inductive