Carbon-13 isotope effect for decarboxylation of phenylpropiolic acid (PPA) in opthophosphoric acid

The carbon-13 isotope effect accompanying decarboxylation of phenylpropiolic acid in 85% orthophosphoric acid has been studied in the temperature range 323-353 K. The first fraction of CO2 was found to be generated at 323 k in 85% H3PO4 with the carbon 13 isotope effect, k12/k13, equal to 1.011-1.015. The subsequent fractions of carbon dioxide produced in the temperature range 323-353 K have shown none but about 0.2% kinetic isotope effect (KIE). It has been suggested that the initial larger carbon-13 fractionation, accompanying the decarboxylation of PPA in 85% orthophosphoric acid, is caused by isotopic equilibria linking decarboxylating monomeric species of PPA with dimeric and larger units formed during solvation and dissolving of solid PPA in orthophosphoric acid. The solvated PPA molecules are decarboxylated in the homogeneous solution with the negligible carbon-13 KIE. The triple bond of PPA is attacked by the hydrated protons, H3O+aq, and formation of the Calpha-H bond is the rate determining step. The bond rupture between the alpha carbon and the carboxyl group is the next fast reaction step. The enthalpy of activation , ΔH = 123.4 kJ/mol, and the positive entropy of activation ΔS +30 e.u. (SI), exclude the chelate like structure of the transition state. This suggests a flexible structure of the transition state complex which must be less solvated than in the initial state. Some further studies of the carbon-13 KIEs in PPA environment are suggested. (author)