Question

Arrange the following in increasing order of $p{K}_a$ values:

• A. $\text{(I) < (II) < (III) < (IV)}$
• B. $\text{(I) < (II) < (IV) < (III)}$
• C. $\text{(IV) < (III) < (II) < (I)}$
• D. $\text{(III) < (IV) < (II) < (I)}$

Answer 1

The correct option is B $\text{(I) < (II) < (IV) < (III)}$


More is the stability of conjugate base, less is the $p{K}_a$ value.
The conjugate base in (I) is most stable due to aromaticity followed by the conjugate base in (II).
The antiaromatic conjugate base in (III) and (IV) will be least stable.
(III) and (IV) both are antiaromatic, but (IV) has more resonating structures than (III) and $p{K}_a$ order is (I) < (II) < (IV) < (III)