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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

A carbanion is stabilized when its lone pair occupies an orbital of higher s-character, because s-orbitals hold electrons closer to the nucleus (lower energy). The acetylide carbon HC≡C(-) is sp hybridized (50% s), the phenyl and vinyl-type carbons are sp2 (33% s), and the neopentyl carbanion carbon is sp3 (25% s). Highest s-character means the acetylide anion is the most stable.

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