Which of the following is the most stable cation

Which of the following is the most stable cation? C=CH2, CH3+, CCH3+, CH3C+, CH3CH2+, CH3CH2CH2+, CH3CH2CH2CH2+ Clear my choice

The Correct Answer and Explanation is:

The most stable cation among the given options is:

CH₃CH₂⁺ (ethyl cation)

Correct Answer: CH₃CH₂⁺

Explanation

Carbocation stability depends primarily on the number of alkyl groups attached to the positively charged carbon atom. The key stabilizing factors are hyperconjugation and inductive effects. More substituted carbocations (secondary, tertiary) are typically more stable than primary or methyl carbocations because they are better stabilized by the electron-donating effects of adjacent carbon atoms.

Let us analyze each option:

1. C=CH₂: This represents a vinyl cation (positively charged carbon directly bonded to a double bond). Vinyl carbocations are highly unstable because the positive charge is on an sp² hybridized carbon and cannot be stabilized by resonance or hyperconjugation.
2. CH₃⁺ (methyl cation): This is a primary carbocation with no alkyl groups for stabilization. It is highly unstable due to lack of hyperconjugation and inductive effects.
3. CCH₃⁺: Assuming this means a tert-butyl cation or a similar structure, we need clarification. If it is the cation formed from CH₃-C⁺-CH₃ (isopropyl cation), it is more stable than methyl but less stable than ethyl.
4. CH₃C⁺: This is an ethyl cation (CH₃–C⁺H₂). It is primary, but it is more stable than the methyl cation due to inductive effects from one methyl group.
5. CH₃CH₂⁺: This is the ethyl carbocation, which is a primary carbocation, slightly more stable than CH₃⁺ due to one alkyl group’s inductive effect.
6. CH₃CH₂CH₂⁺: This is a primary carbocation, just like ethyl, but the positive charge is still on a primary carbon, offering no extra hyperconjugation benefit.
7. CH₃CH₂CH₂CH₂⁺: Again a primary carbocation, similar to the previous one.

Among these, the ethyl cation (CH₃CH₂⁺) is slightly more stable due to a better inductive effect compared to methyl or longer primary chains, which do not offer extra stabilization.