Four carboxylic acids are shown beside the options (the drawn compound is acrylic acid, pKa 4.25, for reference). Which structure is the strongest acid?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: B

Propiolic acid (pKa about 1.9) has an sp carbon attached to COOH; the greater s character makes it strongly electron-withdrawing, stronger than the cyano group in cyanoacetic acid (2.5) and the Cl of chloroacetic acid (2.9). 3-Fluoropropanoic acid (about 4.0) suffers from the extra CH2 separating F from COOH.

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