Hydrolytic Decarboxylation of Carboxylic Acids and the Formation of Protonated Carbonic Acid

Acid-catalyzed decarboxylation reactions of carboxylic acids should avoid formation of protonated carbon dioxide, a very high energy species. A potential alternative route parallels ester hydrolysis, with addition of water to the carboxyl group followed by protonation of the unsaturated leaving group and formation of protonated carbonic acid, a species that had been predicted to be a viable reaction intermediate. The hydrolytic mechanism for the decarboxylation of pyrrole-2-carboxylic acid is consistent with observed (12)C/(13)C kinetic isotope effects (1.010 +/- 0.001 at H(0) = -0.01 and 1.043 +/- 0.001 at H(0) = -2.6), solvent kinetic isotope effects (k(H(2))(O)/k(D(2))(O) = 2 at H(0) = 0.9; k(H(2))(O)/k(D(2))(O) = 1 at H(0) = -2.9), and activation parameters [DeltaH() = 23.5 kcal.mol(-1) and DeltaS() = 5.5 cal.deg(-1).mol(-1) at H(0) = -2.9]. Thus, the specific route for a decarboxylation process is a consequence of the nature of the potential carbanion (or its conjugate acid), the acidity of the medium and avoidance of formation of protonated carbon dioxide.