Carbon-13 Kinetic Isotope Effects in the Decarbonylation Of Liquid Formic Acid in Water Solution

Abstract

Carbon-13 kinetic isotope effects (13C KIEs) in the decarbonylation of formic acid diluted with water 1:1 (V:V) has been studied in the 130–181°C temperature interval in sealed vacuum all glass reactions vessels. The experimental 13C KIEs are higher than the 13C KIEs values extrapolated from low 50–100°C temperature interval 13C KIEs observed in the decarbonylation of water free liquid formic acid but less than 13C KIEs expected theoretically assuming the complete carbon—oxygen (13C‒16O) bond rupture in the transition state. For instance the (k12/k13) KIE found in this study is 1.0375 at 170.2°C while the “full” 13C KIE is expected to be 1.0429 at this temperature. The 13C KIE extrapolated to 170.2°C from the low temperature data published earlier is 1.0299. - The Arrhenius activation energy for the decarbonylation of formic acid in 1:1 water solution E = 31.3 kcal/mol is by 6 kcal/mol higher than that which was obtained with 99,9% pure formic acid (E = 25,660 kcal/mol). The entropy of activation ΔS ≠ increased from -21.4 e.u. (pure formic acid) to -15.3 e.u. in the decarbonylation of formic acid diluted with water 1:1. The increases of the enthalpy of activation, of the entropy of activation and partly of the carbon-13 kinetic isotope effect observed in the decarbonylation of formic acid in water solution have been rationalized by suggesting isotopic equilibria interfering the pure kinetic fractionation of 13C and intervention of the water molecules into the process of transfer of protons to the formic acid molecules lowering the absolute rate of their decomposition.