Rank the following in decreasing order of basic strength is:
A
B > A > D > C
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B
D > A > B > C
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C
A > D > B > C
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D
A > D > C > B
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Solution
The correct option is C A > D > B > C Basic strength∝1Stability of anion
Also, Basic strength∝1Dispersion 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.