Q109 / 200 · MHT CET - Medical 2026 (21 April Shift 1) · Chemistry
Compare the relative stability of given carbocations.
$\text{CH}_3\text{-}\overset{+}{\text{C}}\text{-(CH}_3)_2$ (I)$\qquad$ $\text{CH}_3\text{-}\overset{+}{\text{C}}\text{H-CH}_3$ (II)$\qquad$ $\text{CH}_3\text{-}\overset{+}{\text{C}}\text{H}_2$ (III)
$\text{CH}_3\text{-}\overset{+}{\text{C}}\text{-(CH}_3)_2$ (I)$\qquad$ $\text{CH}_3\text{-}\overset{+}{\text{C}}\text{H-CH}_3$ (II)$\qquad$ $\text{CH}_3\text{-}\overset{+}{\text{C}}\text{H}_2$ (III)
- AIII > II > I
- BII > I > III
- CI > III > II
- Correct answerDI > II > III
Solution
The stability of alkyl carbocations is determined by the +I effect of alkyl groups and hyperconjugation.
Carbocation I is a tertiary ($3^{\circ}$) carbocation with 9 $\alpha$-hydrogens, providing maximum hyperconjugation and +I effect.
Carbocation II is a secondary ($2^{\circ}$) carbocation with 6 $\alpha$-hydrogens.
Carbocation III is a primary ($1^{\circ}$) carbocation with 3 $\alpha$-hydrogens.
More the number of $\alpha$-hydrogens, more is the hyperconjugation and greater is the stability.
Thus, the relative stability order is I > II > III.
Answer: I > II > III
Carbocation I is a tertiary ($3^{\circ}$) carbocation with 9 $\alpha$-hydrogens, providing maximum hyperconjugation and +I effect.
Carbocation II is a secondary ($2^{\circ}$) carbocation with 6 $\alpha$-hydrogens.
Carbocation III is a primary ($1^{\circ}$) carbocation with 3 $\alpha$-hydrogens.
More the number of $\alpha$-hydrogens, more is the hyperconjugation and greater is the stability.
Thus, the relative stability order is I > II > III.
Answer: I > II > III
Hence, the correct answer is option: 'D'.