Question:

Which of the following is the most stable carbocation?
 

Updated On: Apr 7, 2025
  • most stable carbocation
  • most stable carbocation
  • most stable carbocation
  • most stable carbocation
  • most stable carbocation

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The Correct Option is D

Approach Solution - 1

Carbocations are positively charged species with a carbon atom bearing only six electrons. Carbocation stability is influenced primarily by two factors:

Inductive Effect: Alkyl groups are electron-donating groups due to the inductive effect (release of electrons through sigma bonds). The more alkyl groups attached to the carbocation carbon, the more the positive charge is dispersed, and the more stable the carbocation becomes.

Hyperconjugation: This is the stabilizing interaction that results from the interaction of the electrons in a sigma bond (usually C-H or C-C) with an adjacent empty (or partially filled) p-orbital. The more alkyl groups attached to the carbocation carbon, the more hyperconjugation possible, increasing stability.

Order of Carbocation Stability:

Tertiary (3°) > Secondary (2°) > Primary (1°) > Methyl

  • Tertiary (3°): The carbocation carbon is attached to three other carbon atoms.
  • Secondary (2°): The carbocation carbon is attached to two other carbon atoms.
  • Primary (1°): The carbocation carbon is attached to one other carbon atom.

Methyl: The carbocation carbon is attached to no other carbon atoms.

Analyzing the Given Carbocations:

  • (A) CH3-CH2+: This is a primary carbocation.
  • (B) CH3+: This is a methyl carbocation.
  • (C) CH3-CH+-CH3: This is a secondary carbocation.
  • (D) (CH3)3C+: This is a tertiary carbocation.
  • (E) CH3-CH2-CH2+: This is a primary carbocation.

Therefore, the most stable carbocation is (D) (CH3)3C+ (the tertiary carbocation).

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Approach Solution -2

1. Factors affecting carbocation stability:

  • Hyperconjugation: The ability of adjacent C-H bonds to stabilize the positive charge through overlap of \( \sigma \)-orbitals with the empty p-orbital of the positively charged carbon.
  • Inductive Effect: Electron-donating groups (e.g., alkyl groups) stabilize the positive charge by donating electron density.
  • Resonance Stabilization: If the carbocation can be stabilized by resonance, it becomes more stable.

2. Analyze each carbocation:

  • (A) \( \text{CH}_3-\overset{+}{\text{CH}}_2 \):
    • This is a primary carbocation (\( \text{R}^+ \)).
    • It has only one alkyl group (methyl group) attached to the positively charged carbon.
    • Stability: Relatively unstable due to limited hyperconjugation and inductive stabilization.
  • (B) \( \overset{+}{\text{CH}}_3 \):
    • This is a methyl carbocation.
    • It has no alkyl groups attached to the positively charged carbon.
    • Stability: Highly unstable as it lacks any hyperconjugation or inductive stabilization.
  • (C) \( \text{CH}_3-\overset{+}{\text{CH}}-\text{CH}_3 \):
    • This is a secondary carbocation (\( \text{R}_2\text{C}^+ \)).
    • It has two alkyl groups (methyl groups) attached to the positively charged carbon.
    • Stability: More stable than primary carbocations due to increased hyperconjugation and inductive stabilization from the two methyl groups.
  • (D) \( \left(\text{CH}_3\right)_3\overset{+}{\text{C}} \):
    • This is a tertiary carbocation (\( \text{R}_3\text{C}^+ \)).
    • It has three alkyl groups (methyl groups) attached to the positively charged carbon.
    • Stability: Most stable among the given options because it has maximum hyperconjugation and inductive stabilization from the three methyl groups.
  • (E) \( \text{CH}_3-\overset{+}{\text{CH}}_2-\text{CH}_2 \):
    • This is a primary carbocation (\( \text{R}^+ \)).
    • It has only one alkyl group (methyl group) attached to the positively charged carbon.
    • Stability: Similar to option (A), this is relatively unstable due to limited stabilizing groups.

3. Compare stability:

  • Methyl carbocation (B): Least stable due to no stabilizing groups.
  • Primary carbocations (A and E): Unstable due to limited stabilizing groups.
  • Secondary carbocation (C): More stable than primary carbocations but less stable than tertiary carbocations.
  • Tertiary carbocation (D): Most stable due to maximum hyperconjugation and inductive stabilization.

4. Identify the most stable carbocation:

  • The most stable carbocation is the tertiary carbocation (\( \left(\text{CH}_3\right)_3\overset{+}{\text{C}} \)), which corresponds to Option (D).

5. Final answer:

The correct option is (D).

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