Question

Which of the following compounds has the highest value of $p{K}_a$?

• A. $I$
• B. $II$
• C. $III$
• D. $IV$

Answer 1

The correct option is B $II$

Since,
$\text{Acidic strength}\propto \frac{1}{p{K}_a}$
Conjugate base so formed after removal of $H^{+}$ of all compounds is :

Since -I effect of chlorine stabilizes the conjugate base formed, more the chlorine atoms attached, more is the stability of conjugate base and more is the acidic nature of the compound.

In II, there is no chlorine atom attached, and $-C{H}_3$ attached increases the electron density through $+I$ effect, making it the least stable. Hence, it is least acidic and will have highest $p{K}_a$ .
So, II has the highest value of $p{K}_a$