Question

Which of the following is the correct order of stability for the given carbocations?

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

Answer 1

The correct option is B III > II > I

$\text{Stabilty of carbocation}\propto +Ieffect\propto \frac{1}{-Ieffect}$

The priority order of the factors upon which the strength of inductive effect depends is:
Distance > Number > Power

Hence, the $-I$ effect of $-$I group will be the least and therefore, the stability of carbocation (III) will be the highest.
Between $-CN$ and $-COOH$ groups, the $-I$ effect of $-CN$ is more due to the higher electron-withdrawing power of the $-CN$ group than $-COOH$ group.

Therefore, the correct order of stability of carbocations is III > II > I.