Question

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

• A. I < II < III
• B. I < III < II
• C. II < III < I
• D. II < I < III

Answer 1

The correct option is C II < III < I


More is the stability of conjugate base, stronger will be the acidic strength and hence lower will be the $p{K}_a$ value.

The conjugate base in (II) is the most stable due to aromaticity hence it has least $p{K}_a$ value.
The conjugate base of (I) is anti-aromatic and least stable so it has highest $p{K}_a$ value.
Non aromatic compound is more stable than anti-aromatic compound.
$\therefore$ Correct order is: II < III < I