Question

Rank in the order of increasing basic strength.

• A. (d) < (b) < (c) < (a)
• B. (d) < (c) < (b) < (a)
• C. (b) < (a) < (c) < (d)
• D. (c) < (b) < (a) < (d)

Answer 1

The correct option is A (d) < (b) < (c) < (a)

More stable the conjugate acid, more will be its basic strength.
Conjugate acid of (a) is stabilised both $N{H}_2$ present in the molecule thus giving three equivalent resonating structures. In (c), there will be 3 resonating structures, but two are equivalent. Hence (c) is less basic than (a).
In (b), there are 2 equivalent resonating structures so it will be more stable and more basic than (d) where there is no resonance.

Therefore, the increasing order of basic strength follows,
(d) < (b) < (c) < (a)