The thermal decomposition of propanoic and 2-methylpropanoic anhydrides in the gas phase

Abstract

The homogeneous gas-phase thermal decompositions of propanoic anhydride and 2-methylpropanoic anhydride have been studied over the temperature ranges 220–294 and 246–283 °C respectively. Both reactions obey first-order kinetics, are unaffected by chain inhibitors and give the parent acid and methylketen or dimethylketen respectively as sole products.

Kinetic parameters are given by the equations k/s^–1= 10^11.38 exp (–140 400 J mol^–1/RT) in the case of propanoic anhydride, and k/s^–1= 1011.81 exp (–151 500 J mol^–1/RT) for 2-methylpropanoic anhydride. Equilibrium constants were also measured in the propanoic anhydride system and gave ΔH°= 95 095 J mol^–1 and ΔS°= 132.2 J K^–1 mol^–1 in the direction of decomposition. The effect of alkyl substitution on the transition states for anhydride decomposition is discussed.