Question

Arrange the following carbocations in the order of increasing stability.

(a) Benzyl > 3^0 > 2^0 > 1^0

(b) Benzyl > 1^0 > 2^0 > 3^0

(c) 3^0 > 2^0 > 1^0 > Benzyl

(d) 1^0 > 2^0 > 3^0 > Benzyl

This question was posed to me in an internship interview.

I want to ask this question from Carbocations topic in section Reactivity of Molecules Reaction Mechanism of Engineering Chemistry I

Answer 1

The correct answer is:

(a) Benzyl > 3^0 > 2^0 > 1^0

Explanation: Carbocation stability is influenced by several factors, including inductive effects, resonance, and hyperconjugation.

1. Benzyl carbocation (C6H5-CH2+): The benzyl carbocation is very stable due to the resonance delocalization of the positive charge into the aromatic ring. The ability of the aromatic ring to stabilize the carbocation through resonance makes it more stable than other types.

2. Tertiary (3^0) carbocation: Tertiary carbocations are stable because they can benefit from hyperconjugation and inductive effects from the three alkyl groups attached to the positively charged carbon. This provides additional electron density to stabilize the carbocation.

3. Secondary (2^0) carbocation: Secondary carbocations are less stable than tertiary ones, but they still benefit from some hyperconjugation and inductive effects from two alkyl groups.

4. Primary (1^0) carbocation: Primary carbocations are the least stable because they have only one alkyl group to donate electron density through hyperconjugation or induction.

Thus, the stability order is: Benzyl > Tertiary > Secondary > Primary.

Hence, the correct answer is (a) Benzyl > 3^0 > 2^0 > 1^0.