Which is the strongest acid among propanoic acid, prop-2-enoic acid, prop-2-ynoic acid and butanoic acid?

Abutanoic acid
Bprop-2-ynoic acid
Cpropanoic acid
Dprop-2-enoic acid

Answer: B

The carboxyl group is attached to an sp carbon in prop-2-ynoic acid, which is more electronegative than sp2 or sp3 carbon, and so withdraws electrons and stabilises the carboxylate (pKa about 1.9). Prop-2-enoic acid (sp2) has pKa about 4.25 and propanoic and butanoic acids (sp3, +I alkyl) about 4.9.

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