Carbon-13 isotope fractionation in the decarboxylation of phenylpropiolic acid (PPA) below and above its melting point and in the decarboxylation of PPA in phenylacetylene medium

C-13 isotope fractionation in the decarboxylation of pure phenylpropiolic acid (PPA) below and above its melting point and the decarboxylation of PPA in phenylacetylene solutions has been investigated in sealed under vacuum reaction vessels. The reactive PPA undergoing decarboxylation polymerizes with the liquid product, phenylacetylene in reaction cage producing a condensation compound, which does not decarboxylate measurably in the 120-190 degrees C. Especially low final carbon dioxide yields (about 11%)have been obtained in the decarboxylation of PPA in phenylacetylene solution at 132 degrees C and below this temperature. The carbon dioxide is depleted in carbon-13. The ratio of the carbon isotope ratios of carboxylic carbon of PPA before decarboxylation, R(C-13/C-12)(so), and of the first portions of carbon dioxide obtained at partial decarboxylations, R(C-13/C-12)(pf), located in the range 1.007-1.010, indicates that the pure kinetic fractionation of C-13 in the elementary decarboxylation step is negligible and the C-13 fractionation in the condensed phase dimer/monomer equilibria contributes mainly to the resultant experimental carbon isotope fractionation. A preliminary discussion of the experimental isotope findings is presented.