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 a cid (PPA) below and above its melting point and the decarboxylation of PPA in phenylacetylene solutions has been investigated in sealed under vacuum r eaction vessels. The reactive PPA undergoing decarboxylation polymerizes wi th the liquid product, phenylacetylene in reaction cage producing a condens ation compound, which does not decarboxylate measurably in the 120-190 degr ees C. Especially low final carbon dioxide yields (about 11%)have been obta ined in the decarboxylation of PPA in phenylacetylene solution at 132 degre es C and below this temperature. The carbon dioxide is depleted in carbon-1 3. The ratio of the carbon isotope ratios of carboxylic carbon of PPA befor e decarboxylation, R(C-13/C-12)(so), and of the first portions of carbon di oxide obtained at partial decarboxylations, R(C-13/C-12)(pf), located in th e 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 fractiona tion in the condensed phase dimer/monomer equilibria contributes mainly to the resultant experimental carbon isotope fractionation. A preliminary disc ussion of the experimental isotope findings is presented.