Carbon-13 isotope effect for decarboxylation of phenylpropiolic acid (PPA) in concentrated phosphoric acids

13C isotope effects on the decarboxylation of phenylpropiolic acid (PPA) in 93–97% H3PO4 and in 23% diphosphoric acid in orthophosphoric acid have been investigated from 293 to 353 K. The initial 13C fractionations in all three systems are in agreement with the 13C decarboxylation fractionations expected assuming the loss of the one carbon-carbon bond in the transition state. In 100% H3PO4 and in 23% diphosphoric acid in orthophosphoric acid later fractions of carbon dioxide are depleted in carbon-13 to a greater degree than expected, probably due to isotopic preequilibria between stable and decarboxylating forms of PPA. At 353 K and higher temperature strong deviations of the experimental rate constants and of the experimental 13C fractionations from the values extrapolated from lower temperature are observed. A tentative decarboxylation scheme operating in concentrated phosphoric acid media is proposed.