A contribution to carbon-13 and carbon-14 kinetic isotope effect studies in the decarbonylation of oxalic acid in sulphuric and phosphoric acids

Carbon-13 intramolecular kinetic isotope effects (K.I.E.) in the decarbonylation of oxalic acid in concentrated sulfuric acid, in sulfuric acid diluted with water and in fuming sulfuric acid have been reinvestigated or investigated. Intramolecular and intermolecular 13C K.I.E. in the decarbonylation of oxalic acid of natural isotopic composition in phosphoric acids media have been initiated also. The previous determinations of 13C K.I.E. in the decarbonylation of oxalic acid in concentrated sulfuric acid at 100 C have been reproduced and it has been shown that the dilution of sulfuric acid with water as well as increase of the concentration of SO3 over the stoichiometric one (influencing remarkably the rates of decarbonylation) do not change significantly the 13C fractionation in this reaction contrary to the large medium and temperature effects (especially at 100 C) observed in the decarbonylations of formic and lactic acids in sulfuric and phosphoric acid media. The above results have been rationalized by strengthening the view that in the decarbonylation of oxalic acid all bonds are substantially altered in T.S. but the loss of two single C-C and C-O isotopic bonds in T.S. is compensated by the increase of bonding between carbon and oxygen in the carbonyl group which undergoes the transformation into free carbon monoxide molecule and diminishes the 13C fractation. Additional isotope effect data have been provided which show that the structure of the T.S. in the decarbonylation of bifunctional oxalic acid in sulfuric acid and in phosphoric acids media differs largely from the structure of the T.S. in the decarbonylation of formic and lactic acids in sulfuric and phosphoric acids. (author). 22 refs, 2 tabs