Gas phase dissociation of protonated acetic acid to the acyl cation and water. Heat of formation of CH3CO+ and proton affinity of ketene

Abstract

Pulsed electron beam high pressure mass spectrometry was used to measure the enthalpy and entropy of the dehydration of protonated acetic acid, and the proton affinity of ketene in the gas phase. The variation of the equilibrium constant with temperature from 515 to 615 K led to ΔH⁰ of 24.6 kcal/mol and a ΔS⁰ of 33.1 cal/deg for the dehydration of protonated acetic acid. Proton transfer equilibria measurements of reactions: BH⁺ + CH₂CO = B + CH₃CO⁺ at 600 K with reference bases of known proton affinity led to a proton affinity of 195 kcal/mol for ketene. The heat of formation of the acetyl cation (ΔH_f⁰(CH₃CO⁺) = 160) was obtained from both the proton affinity of ketene, and from the dehydration enthalpy of protonated acetic acid combined with the proton affinity of acetic acid. The energetics for the dehydration reaction in the gas phase and in solution are compared.