Question

The increasing order of the $p{K}_a$ values of the following compounds is :

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• A. $B<C<D<A$
• B. $D<A<C<B$
• C. $B<C<A<D$
• D. $C<B<A<D$

Answer 1

The correct option is C $B<C<A<D$

Acidic strength is inversely proportional to $p{K}_a$ value.

Acid strength is directly proportional to the stability of conjugate base.

The conjugate base formed by (B) is stabilised by -R(resonance) effect of $N{O}_2$ group. Hence, it is most acidic.
The conjugate base formed by (C) is stabilised by -I(inductive) effect of $N{O}_2$ group. Hence, it is less acidic than (A) but more acidic than (C) and (D).
Conjugate base of (D) is destabilised by +R effect of OMe group. Hence, it is the least acidic compound of all.
Hence, the correct order of acidity is
$B>C>A>D$

Thus, the correct order of $p{K}_a$ is