Carbon-13 isotope effect in the decarboxylation of phenylpropiolic acid insulphuric acid

C-13 isotope effect in the decarboxylation of phenylpropiolic acid in dilut ed aqueous sulphuric acid and in its concentrated solution has been studied in vacuum reaction vessels. The (k(12)/k(13)) KIEs are located in the rang e 1.0042-1.0046 in the temperature interval 90-110 degrees C in the case of decarboxylation of PPA in H2SO4 diluted with water. C-13 KIEs in the decar boxylation of PPA in the concentrated H2SO4 were found to be 1.060-1.050 be tween 16-60 degrees C. A discussion of the negligible C-13 KIEs in the deca rboxylation of PPA in diluted aqueous sulphuric acid (1:4/V:V/H2SO4:H2O) ha s been given and the discussion of the large C-13 KIE, exceeding the values of C-13 KIE calculated for this temperature interval, assuming the "full" C-*C bond rupture in the T.S., has been presented. The kinetic parameters, deduced from the temperature dependence of the absolute rate constants of d ecarboxylation reaction and the C-13 KIE data, have been interpreted as the result of the loose T.S. in the case of decarboxylation of PPA in aqueous solution of H2SO4 and as the result of the "chelate-like" T.S. in the decar boxylation of PPA in concentreted sulphuric acid. The protonation of the tr iple bond and/or formation of he C-alpha-H bond are the rate and the C-13 K IE determining processes in 3.75 M H2SO4, while in the case of decarboxylat ion of PPA in concentrated H2SO4, the phenylpropiolic acid is fully protona ted and the rupture of the C-*C bond, preceded by preequilibria, determines the large C-13 isotope effect observed.