Carbon-13 kinetic isotope effects in the decarbonylation of liquid formic acid in water solution

Carbon-13 kinetic isotope effects (C-13 KIEs) in the decarbonylation: of Fo rmic acid diluted with water 1:1 (V:V) has been studied in the 130-181 degr ees C temperature interval in sealed vacuum all glass reactions vessels. Th e experimental C-13 KIEs are higher than the C-13 KIEs values extrapolated from low 50-100 degrees C temperature interval C-13 KIEs observed in the de carbonylation of water free liquid formic acid but less than C-13 KIEs expe cted theoretically assuming the complete carbon-oxygen (C-13 -O-16) bond ru pture in the transition state. For instance the (k(12)/k(13)) KIE found in this study is 1.0375 at 170.2 degrees C while the "full" C-13 KIE is expect ed to be 1.0429 at this temperature. The C-13 KIE extrapolated to 170.2 deg rees C from the low temperature data published earlier is 1.0299.- The Arrh enius 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 obt ained with 99.9% pure formic acid (E = 25,660 kcal/mol). The entropy of act ivation Delta S-not equal 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 decarbonylat ion of formic acid in water solution have been rationalized by suggesting i sotopic equilibria interfering the pure kinetic fractionation of C-13 and i ntervention of the water molecules into the process of transfer of protons to the formic acid molecules lowering the absolute rate of their decomposit ion.