Question

Rank the following in decreasing order of basic strength is:

• A. B > A > D > C
• B. D > A > B > C
• C. A > D > B > C
• D. A > D > C > B

Answer 1

The correct option is C A > D > B > C

$\text{Basic strength}\propto \frac{1}{\text{Stability of anion}}$
Also,
$\text{Basic strength}\propto \frac{1}{\text{Dispersion of negative charge}}$
In case of C :
There are two equivalent resonating structures, so it is the most stable species, and hence the least basic.


Between A , B and C:
In A the negative charge is on the most electropositive atom i.e. carbon, as compared to B and C where negative charge is on sulphur and oxygen, which are more electronegative.
So, availability of negative charge is the highest in A, and hence it is the most basic.
Between B and D
Sulphur being bigger in size as compared to oxygen, dispersion of negative charge is more in sulphur, and hence B is less basic than D
Correct order :
A > D > B > C

Note: we can also solve this by comparing the acidic strength of respective conjugate acids and then reversing the order for basic strength.