Among the following, the hydrogen attached to carbon is most acidic in

A $HC \equiv CH$
B $CH X_{2} = CH X_{2}$
C $CH X_{3} - CH X_{3}$
D $CH X_{4}$

Answer: A

The conjugate base of ethyne has the lone pair in an sp orbital (50 percent s-character), which holds the electron pair closest to the nucleus and stabilises the carbanion best. Ethene (sp2) and the alkanes (sp3) are much less acidic.

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