Carbon isotope effect studies in the mechanism of reactions of alkynes with formic acid. II. 13C fractionation in the decarbonylation of Merck formic acid in the presence of phenylacetylene

The 13C fractionation has been studied in the reaction of phenylacetylene with the excess of liquid Merck formic acid at 30 and 40 deg C to see the contribution of the 13C fractionation in the formolysis of transient α-formoxystyrene to the experimentally observed global 13C fractionation. The 13C fractionation has been investigated also in the hydration of 1 ml of PhC ≡ CH with 1 ml of formic acid in the temperature interval of 80-100 deg C. The 13C KIE equal to 1.0168 at 91.75 deg C and 1.0167 at 100 deg C indicate that the self-decomposition of formic acid in such experimental conditions is already largely suppressed. The isotope effect is discussed within the framework of the sequence of reaction steps leading to acetophenone and carbon monoxide production listed in part I. (author)