JEE Chemistry 2024 Question with Solution

Given: The stability order of carbocations is to be identified.

Find: The correct order of stability among the given carbocations.

Carbocation stability depends mainly on hyperconjugation and the +I effect of alkyl groups. The more alkyl groups attached to the positively charged carbon, the more stable the carbocation.

Thus, the general order is:

$$(\text{CH}_3)_3\text{C}^+ > (\text{CH}_3)_2\text{CH}^+ > \text{CH}_3\text{CH}_2^+ > \text{CH}_3^+$$

So the stability order is tertiary > secondary > primary > methyl.

The solution explicitly states that the correct option is B, although the listed option texts appear inconsistent with the worked order. Based on the solution, the correct option is B.

Given: Carbocations of different degrees of substitution.

Find: Which one is most stable and the resulting order.

Key idea: Alkyl groups stabilize a carbocation by donating electron density through the inductive effect and by hyperconjugation.

1. A tertiary carbocation has three alkyl groups, so it gets maximum stabilization.
2. A secondary carbocation has two alkyl groups, so it is less stable than tertiary.
3. A primary carbocation has only one alkyl group, so its stabilization is still lower.
4. A methyl carbocation has no alkyl group, so it is least stable.

Hence,

$$(\text{CH}_3)_3\text{C}^+ > (\text{CH}_3)_2\text{CH}^+ > \text{CH}_3\text{CH}_2^+ > \text{CH}_3^+$$

Therefore, the correct option according to the solution is B.