Thermal decomposition of 2-fluoroethanol: single pulse shock tube and ab initio studies

Abstract

The thermal decomposition of 2-fluoroethanol (FEOH) has been studied in the temperature range of 1000-1200 K behind reflected shock waves in a single pulse shock tube. The total pressures behind the reflected shocks varied between 13 and 23 atm. The products observed in the covered temperature range are CH₃CHO, C₂H₃F, CH₄, CO, C₂H₄, and C₂H₆. The unimolecular eliminations of HF and H₂O are found to be the major channels through which 2-fluoroethanol decomposes under these conditions. The rate constant for HF elimination is found to be 10^13.17±0.33 exp[-(59.5 ± 1.7)/(RT)] s^-1, and the rate constant for H₂O elimination is found to be 10^14.30±0.34 exp[-(69.7 ± 1.7)/(RT)] s^-1, where the activation energies are given in kcal mol^-1. The CH₃CHO produced by HF elimination through the vinyl alcohol intermediate is chemically active and decomposes leading to CH₄ and CH₃CH₃ products. The production of ethylene could not be explained from the CH₃CHO pyrolysis mechanism. It is most likely formed directly from the reactant through HOF elimination or by radical processes beginning with C-O bond dissociation. Ab initio (Hartree-Fock [HF] and second-order Møller-Plesset perturbation theory [MP2]) and density functional theory [DFT] calculations have been carried out to find the transition state and activation barrier for HF and H₂O elimination reactions. The HF calculations overestimate the barrier by 18 kcal mol^-1 for HF elimination and 22 kcal mol^-1 for H₂O elimination, and including electron correlation improves the agreement. In particular, DFT predictions for activation energies for HF and H₂O elimination reactions are within 1 kcal mol^-1 of the experimental values.