Question

Find the correct order of basic strength of the following compounds.

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

Answer 1

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

Here compound (c) is most basic because its lone pair of electrons are readily available for donation. They are not involved in any resonance. Whereas in (a), (b) and (c) the lone pair on nitrogen is involved in resonance.
Here compound (b) form 5 resonating structures by delocalisation of lone pair of N in phenyl ring and (a) forms two resonating structure by delocalisation of lone pair on both side of double bond and (d) forms one resonating structure.

More the resonating structure, more the stability. Hence, lesser the basic strength.
Hence the increasing order of basic strength is
(b) < (a) < (d) < (c)