Carbon-13 Isotope Fractionation in the Decarboxylation of Phenylpropiolic Acid in Orthophosphoric Acid and in Diphosphoric Acid

C-13 isotope fractionation in the decarboxylation of phenylpropiolic acid (PPA) in the 10% solution of diphosphoric acid (H4P2O7) in orthophosphoric acid (H3PO4) and in the solution of diphosphoric acid in metaphosphoric acid (HPO3) has been studied between 20–140 C (293.16–413.16). The initial C fractionation observed in the decarboxylation of PPA in 10% solution of H4P2O7 in H3PO4 in the temperature interval 20–30 C coincides with (k12/k13) KIE = 1.045, calculated assuming the “full” C–C bond rupture in the decarboxylation transition state. The subsequent carbon dioxide probes collected between 40–70 C are depleted in carbon-13 by about 1% more than the carbon-13 isotope effects calculated for this higher temperature interval. The total yield of carbon dioxide was 96% only. The fast polymerization of phenylpropiolic acid accompanying the decarboxylation of this acid in the solution of pyrophosphoric acid in metaphosphoric acid decreased the total yield of carbon dioxide probes, collected between 40–140 C (so far investigated) to 33%. The mass spectrometric determinations of the isotopic compositions of these probes (Rpf) showed that they are enriched in C with respect to the “initial C/C carbon isotope ratio Rso” for carboxylic carbon of the starting PPA used in the decarboxylation experiments. The last small fractions of CO2, collected at higher temperatures, were produced with the apparent reversed C KIE, (k12/k13) < 1.0.