Introduction to Studies of Deuterium Isotope Effects on the Decarboxylation 13C KIE and on the Decarboxylation of the Phenylpropiolic Acid (PPA) in D3PO4/D2O Solution

Carbon-13 kinetic isotope effect (KIE) in the decarboxylation of phenylpropiolic acid (PPA) in 85% D3PO4 in D2O has been studied and compared with the 13C KIE in decarboxylation of PPA in 85% H3PO4 in H2O. The decarboxylation of PPA in 85% D3PO4 in D2O solution proceeds with slower rate than in 85% H3PO4 solution in H2O in agreement with the proposed mechanism of decarboxylation, which involves the protonation of the triple bond of PPA in the rate determining step. The very negligible decarboxylation 13C KIE equal: (k12/k13)D = 1.0006 0.0012 in 85% D3PO4/D2O medium in the 70–90 graduate C interval, corroborates strongly the previous conclusions drawn from (k12/k13) values found in 85% H3PO4 water solutions. Formation of the sp2(C-D) bond in the decarboxylation transition state inD3PO4/D2Osolutions takes place with less steric requirements than the sp2(C–H) bond formation in H3PO4/H2O medium.