Carbon-13 kinetic isotope effects in the decarbonylation of liquid formic acid of natural isotopic composition initiated by phosphorus pentoxide

The isotopic composition of the consecutive fractions of carbon monoxide produced in the decarbonylation of liquid formic acid of natural isotopic composition initiated by addition of phosphorus pentoxide has been measured in the temperature interval 19–100°C and the observed gradual decrease of the _PDB values and the increase of thek ₁₂/k ₁₃ ratio of the isotopic specific rate constants (KIE values) for each next fraction of CO have been interpreted in terms of conclusions presented in the first paper from this series¹ concerning the decarbonylation of HCOOH (F.A.) in concentrated and diluted with water phosphoric acid media. The initial fast dehydration of F.A. by phosphoric anhydride, P₂O₅, proceeds at room temperture with about 1% carbon-13 KIE. The (k ₁₂/k ₁₃) values increase with time, as the decarbonylation slows down due to the hydration of phosphorus pentoxide with water generated in dehydration of HCOOH and reach the plateau values characteristic for each reaction temperature. These increasing very slowly with reaction times at intermediate temperatures maximum values of (k ₁₂/k ₁₃) ratios are quite close to values of¹³C KIE observed in the decarbonylation of pure F.A. (k ₁₂/k ₁₃=1.0443 at 81°C). Addition of water to liquid F.A. at 90°C and at 100°C caused the further increase of the¹³C KIE. The detailed discussion of the¹³C KIE in the HCOOH–P₂O₅ system has been given.