The nature of the transition state in ester pyrolysis. Part II. The relative rates of pyrolysis of ethyl, isopropyl, and t-butyl acetates, phenylacetates, benzoates, phenyl carbonates, and N-phenylcarbamates

Abstract

Rates of pyrolysis of the title compounds have been measured over not less than a 50° range for each, at temperatures between 522 and 606 K, and the relative rates at 600 K determined. The relative reactivities follow the order: acetates < phenylacetates < benzoates < carbamates < carbonates and the change in the ratios k_Bu^t/k_Prⁱ and k_Prⁱ/k_Et for each series of compounds increases in the same sequence, so that along this series the extent of polarisation in the transition state increases. The increase in this rate spread with increasing reactivity of the ester type arises mainly from the difference in rates of the ethyl and isopropyl compounds and this trend is maintained through to halide pyrolysis. The difference in the transition state polarities for primary and secondary ester pyrolysis is therefore less than that between the secondary and tertiary ester transition states and these differences diminish with increasing reactivity of the ester type. These generalisations do not apply to pyrolysis of ethyl and isopropyl N-phenylcarbamates which are confirmed by the Arrhenius data as decomposing by more than one mechanism.