Carbocation Stability
The correct order of stability of the following carbocations is: (I) CH₃⁺ (II) (CH₃)₂CH⁺ (III) (CH₃)₃C⁺ (IV) CH₃CH₂⁺
Select the correct option:
Solution
III > II > IV > I
Carbocation stability increases with the degree of substitution due to two effects: (1) Inductive effect — alkyl groups are electron-donating (+I), pushing electron density toward the electron-deficient carbon and reducing the charge. (2) Hyperconjugation — adjacent C–H sigma bonds overlap with the empty p orbital on the carbocation, delocalizing the positive charge. (CH₃)₃C⁺ is a tertiary carbocation (most stable), (CH₃)₂CH⁺ is secondary, CH₃CH₂⁺ is primary, and CH₃⁺ is a methyl carbocation (least stable, no alkyl groups). Therefore: III > II > IV > I.
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About This Question
- Subject
- chemistry
- Chapter
- some basic principles of organic chemistry
- Topic
- carbocation stability
- Difficulty
- Medium
- Year
- 2025
Solution
Correct Answer:
III > II > IV > I
Carbocation stability increases with the degree of substitution due to two effects: (1) Inductive effect — alkyl groups are electron-donating (+I), pushing electron density toward the electron-deficient carbon and reducing the charge. (2) Hyperconjugation — adjacent C–H sigma bonds overlap with the empty p orbital on the carbocation, delocalizing the positive charge. (CH₃)₃C⁺ is a tertiary carbocation (most stable), (CH₃)₂CH⁺ is secondary, CH₃CH₂⁺ is primary, and CH₃⁺ is a methyl carbocation (least stable, no alkyl groups). Therefore: III > II > IV > I.
This medium difficulty chemistry question is from the chapter some basic principles of organic chemistry, covering the topic of carbocation stability. It appeared in the 2025 exam.
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