Among the following carbanions or anions, the weakest base is (pKa of conjugate acids: ethane about 50, ethyne 25, ammonia 38, ethene about 44)

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

Answer: C

The conjugate acid of

HC \equiv C X^{-}

is ethyne (pKa 25), the strongest acid in the list, so its conjugate base is the weakest base. The sp carbon holds the negative charge in an orbital with 50% s-character, which stabilises it.

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Among the following carbanions or anions, the weakest base is (pKa of conjugate acids: ethane about 50, ethyne 25, ammonia 38, ethene about 44)

Answer: C

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