JEE Main 2024 · 30 January · evening · Single correct · +4 / −1
The correct stability order of carbocations is
- (a) (CH₃)₃C⁺>CH₃-C(+)H₂>(CH₃)₂C(+)H>C(+)H₃
- (b) (CH₃)₃C(+)>(CH₃)₂C(+)H>CH₃-C(+)H₂>C(+)H₃
- (c) C(+)H₃ > CH₃-C(+)H₂ > CH₃-C(+)H > (CH₃)₃ C⁺
- (d) C(+)H₃>(CH₃)₂C(+)H>CH₃-C(+)H₂>(CH₃)₃C(+)
Answer and solution
Answer (b) (CH₃)₃C(+)>(CH₃)₂C(+)H>CH₃-C(+)H₂>C(+)H₃
Solution
Chemistry ma’am · 3 steps
Count the methyl groups on the positive carbon: each one feeds it electrons and makes it steadier.
- Why alkyl groups help. A carbocation has a carbon with only six electrons and a positive charge. Each alkyl group attached to it donates electron density by the +I effect and by hyperconjugation. This spreads the charge out and stabilises the ion.
- Hyperconjugation count. Hyperconjugation uses the C–H bonds on carbons next to the positive carbon (the α-hydrogens). More α-hydrogens give more resonance-like structures, so the carbocation is more stable.
Carbocation Type Alkyl groups on C⁺ α-hydrogens (CH₃)₃C⁺ tertiary (3°) 3 9 (CH₃)₂CH⁺ secondary (2°) 2 6 CH₃CH₂⁺ primary (1°) 1 3 CH₃⁺ methyl 0 0 - So. The stability order is 3° > 2° > 1° > methyl, that is (CH₃)₃C⁺ > (CH₃)₂CH⁺ > CH₃CH₂⁺ > CH₃⁺, which is option (b).
The idea to keep. For simple alkyl carbocations, stability follows 3° > 2° > 1° > methyl. Count the α-hydrogens: more hyperconjugation and more +I donation mean a more stable cation.
Answer from the NTA final answer key. Solution by PrepWiser.