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Which of the following carbanions is the most stable?

1. HC≡C⁻
2. C6H5⁻
3. (CH3)3C-CH2⁻
4. (CH3)2C=CH⁻

Correct answer: HC≡C⁻

Solution

Carbanion stability is governed by the s-character of the carbon bearing the negative charge. HC≡C⁻ is sp hybridized (50% s-character), making it the most stable. C6H5⁻ (sp², 33%) is next, then vinyl-type (CH3)2C=CH⁻ (sp²), and the neopentyl-type carbanion (CH3)3C-CH2⁻ (sp³, 25%) is least stable.

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