C-13 KINETIC ISOTOPE EFFECTS IN THE DECARBONYLATIONS OF LACTIC-ACID CONTAINING C-13 AT THE NATURAL ABUNDANCE LEVEL

The C-13 kinetic isotope fractionation in the decarbonylation of lactic acid of natural isotopic composition by sulfuric acid has been studied in the temperature range of 20-80-degrees-C. The C-13(1) isotope separation in the decarbonylation of lactic acid by concentrated sulfuric acid depends strongly on the temperature above 40-degrees-C. Below this temperature the C-13 isotope effect in the decarbonylation of lactic acid by concentrated sulfuric acid is normal similarly as has been found in the decarbonylation of lactic [1-C-14] acid. The experimental values of k(C-12)/k(C-13) ratios of isotopic rate constants for C-12 and C-13 are close to, but slightly higher than theoretical C-13-kinetic isotope effects calculated (neglecting tunneling) under the asumption that the C(1)-OH bond is broken in the rate-controlling step of the dehydration reaction. Dilution of concentrated sulfuric acid with water up to 1.4 molar (H2O)/(H2SO4) ratio caused the increase of the C-13 isotope fractionation from 1.0273 found in concentrated sulfuric acid at 80.5-degrees-C to 1.0536 +/- 0.0008 (at 80.6-degrees-C). A discussion of the abnormally high temperature dependence of C-14 and C-13 isotope fractionation in this reaction and the discussion of the problem of relative C-14/C-13 kinetic isotope effects is given.