Question

Among the following, the most stable carbocation is

• A. \(\text{CH}_3^+\)
• B. \((\text{CH}_3)_2\text{CH}^+\)
• C. \(\text{CH}_3\text{-CH}_2^+\)
• D. \((\text{CH}_3)_3\text{C}^+\)

Hint:

More alkyl groups on the positive carbon give more hyperconjugation and inductive stabilisation.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
A carbocation is stabilised when the positive carbon is surrounded by alkyl groups. They donate electron density through the \(+I\) effect and through hyperconjugation.

Step 2: Compare the ions:
\(\text{CH}_3^+\) has no alkyl group, so it has no hyperconjugation at all and is least stable.
\(\text{CH}_3\text{CH}_2^+\) (primary) has 3 alpha hydrogens.
\((\text{CH}_3)_2\text{CH}^+\) (secondary) has 6 alpha hydrogens.
\((\text{CH}_3)_3\text{C}^+\) (tertiary) has 9 alpha hydrogens, the most hyperconjugation and the strongest \(+I\) effect.

Step 3: Order:
\[ 3^\circ > 2^\circ > 1^\circ > \text{methyl} \]
So the tert-butyl cation is most stable, option D.

Final Answer:
The tertiary carbocation \((\text{CH}_3)_3\text{C}^+\) is the most stable, option (D). \[ \boxed{(\text{CH}_3)_3\text{C}^+} \]