TEMPERATURE-DEPENDENCE OF THE C-13 ISOTOPE EFFECT IN THE DECARBONYLATION OF CONCENTRATED FORMIC-ACID SOLUTIONS OF SODIUM-CHLORIDE AND OF METAPHOSPHORIC ACID REAGENT

The temperature dependence of the carbon-13 kinetic isotope effect (C- 13 KIE) in the decarbonylation of concentrated formic acid solutions o f NaCl and of metaphosphoric acid reagent has been determined in the t emperature interval 50-110 degrees C and compared with the C-13 KIE ob served in the decarbonylation of pure liquid formic acid. The (k(12)/k (13)) values lay in the range 1.058-1.047, expected assuming that the C-O vibration is lost in the course of activation of formic acid molec ules leading to decarbonylation. Metaphosphoric acid reagent present i n liquid formic acid at saturation concentrations eliminates all side ''minor'' processes of physical and chemical nature which caused the s light decrease of the experimental C-13 kinetic fractionation observed in the course of decarbonylation of pure formic acid from the expecte d values of C-13 KIE. Sodium chloride is a less effective breaker of t he structure of liquid formic acid than the metaphosphoric acid reagen t.