Acid catalysis in the gas phase: dissociative proton transfer to formate and acetate esters

The flowing afterglow and selected ion flow tube techniques are employed in gas-phase measurements of the intrinsic kinetics of protonation of methyl formate, n-propyl formate, ethyl acetate, and n-propyl acetate and subsequent fragmentation according to[Formula: see text]with R = H and CH3, R′ = CH3, C2H5, and (CH2)2CH3, and A = H2, CH4, CO, and H2O. Protonation by the acids, AH+, with relative strengths spanning a range of 65 kcal mol−1, is observed to proceed extremely rapidly with rate constants at 299 ± 2 K encompassing values of 2.9 to 8.5 × 10−9 cm3 molecule−1 s−1. Fragmentation is observed for HCOOCH3 only with the strongest acid, H3+, to produce CH3OH2+. For HCOO(CH2)2CH3, fragmentation is observed to produce C3H7+ with H3O+, and also HCOOH2+ with H3+. Little fragmentation of CH3COOC2H5 occurs with H3O+ but with H3+ the major product is CH3COOH2+ with smaller amounts of CH3CO+ and C2H5+. Proton transfer from H3O+ to CH3COO(CH2)2CH3 results in considerable dissociation to form CH3COOH2+. The fragmentation of these esters is discussed in terms of known reaction energetics and in terms of mechanisms for unimolecular acyl–oxygen, AAc1, and alkyl–oxygen, AA11, fission often invoked for analogous reactions in solution as well as modifications of these mechanisms which have been proposed in the context of recent gas-phase measurements.