Carbon-13 kinetic isotope effects in the decarbonylation of liquid formic acid assisted with acetic anhydride

The kinetic isotope effect (KIE) in the decarbonylation of formic acid(FA) assisted with acetic anhydride (AA) has been studied in the temperature int erval 18-100 degrees C. The C-13 KIE lying in the range from 1.0541 (at 18. 4 degrees C) to 1.0463(at 100.1 degrees C) indicates that the rupture of th e carbon-oxygen single bond is the reaction step which determines the C-13 fractionation in the observed production of carbon monoxide from formic aci d assisted with Ac2O similarly to what has been observed in the decarbonyla tion of pure liquid formic acid. The magnitude of the C-13 KIE (smaller tha n the value expected for "full C-O bond rupture") indicates again that ther e are intrinsic side processes lowering the C fractionation in the course o f decarbonylations of formic acid. The mechanism of decarbonylation of FA i n the liquid medium consisting initially of "HCOOH-Ac2O" has been discussed and the most probable main and side minor reaction channels have been sele cted. The mechanistic conclusions have been corroborated by presenting the supplementary C-13 KIE data for CO formation in a "HCOOH-Ac2O-EDS" system, where the "overall C-13 KIE" is observed (EDS = etidronate disodium). The p resented isotope data and their interpretation contribute to the full under standing of the decarbonylation of formic acid.