Question

Which of the following acid has the highest $K_a$ value?

• A.
• B.
• C.
• D.

Answer 1

The correct option is B

$\text{Acidic Strength}\propto K_a\propto \frac{1}{p{K}_a}$

Thus, compound which has more acidic nature will have the highest $K_a$ value.

$\text{Acidic Strength}\propto -I\propto \frac{1}{+I}$

Compound (a) has $C{H}_3$ group which has +I effect, thus, it will increase the electron density on conjugate base and reduce its stability. Hence, it is relatively a weak acid.

Compound (c) and (d) has cyclohexane group, which increase the electron density and reduce the stability of conjugate base. The $I$ and $Cl$ groups present in cyclohexane is too far from carboxylic group so its $-I$ effect does not play a effective role here. Hence, they are relatively weak acids.

In compound (b), two $Cl$ atoms are attached to the adjacent carbon. $Cl$ goup shows $-I$ effect, thus, it will reduce the electron density and stabilise the conjugate base formed. Hence, it is the most acidic compound.

$\therefore$ Compound (b) has the highest $K_a$ value.