Which of the following carbanions is the most stable?

A $CH X_{2} = CH X^{-}$
B $HC \equiv C X^{-}$
C $CH X_{3} CH X_{2} X^{-}$
D $(CH X_{3}) X_{2} CH X^{-}$

Answer: B

Stability of a carbanion increases with s-character of the orbital holding the lone pair. The orbital is sp3 (25% s) in the alkyl anions, sp2 (33%) in the vinyl anion and sp (50%) in the ethynyl anion, so the ethynyl anion is the most stable.

Practice General Organic Chemistry I: Electronic Effects More on Applications: stability of carbocations, carbanions, free radicals

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