The correct answer is E.
The less stable an unshared pair of electrons, the likelier it is to
extract a proton from other molecules around, and thus the more basic the
compound. This question about the order of basicity can therefore best be
approached by examining the expected stability of the lone electron pair on
the nitrogen atom in the amino group of the molecules shown. The stability
of the electron pair will be affected by the nature of the other substituent
group present, specifically in terms of whether it is electron-withdrawing
or electron-donating (either inductively or by resonance) and how effective
its influence is based on its position relative to the amino group. The first
thing to notice is that aniline itself (choice D) is a rather weak base because
the electron pair is stabilized via delocalization through the cloud of
the phenyl group (i.e. by resonance) :
One would expect that electron-withdrawing groups would lead to a further
stabilization (and therefore weakening) of the base. Choices A and C, which
contain the electronegative chlorine, would make the compound less basic,
and are therefore not correct. Choice B, which has the nitro group at the meta position, would also stabilize the
electron pair inductively, even though not as effectively as it would if it
were at the o- or p- position,
where it could also stabilize through resonance. Choice E, however, with the
methoxyl group at the para position,
is destabilizing because of the highly unfavorable resonance contributions:
The proximity of the electron pair of nitrogen and the electron density
released by the methoxyl group causes the compound to be more basic than aniline
itself, overwhelming the inductive effect of the electronegative oxygen atom.
Choice E is the most basic, and hence the correct answer.
(A)
Distortion. The chlorine pulls electron density
away from nitrogen and makes it less basic.
(B)
Distortion. The nitro group pulls electron
density away from nitrogen and makes it much less basic.
(C)
Distortion. The chlorine pulls electron density
away from nitrogen and makes it less basic.
(D)
Distortion. Aniline is less basic than choice
E because the nitrogen has less electron density and therefore is less likely
to pick up a proton.