GAS-PHASE PYROLYTIC REACTIONS. PART 3. HAMMETT ρ CONSTANTS OF GAS-PHASE ELIMINATIONS OF ALKYLARYLCARBOXYLATE AND ETHANOATE ESTERS AND THEIR CORRELATION WITH SUBSTRATE MOLECULAR FRAMEWORK

Article abstract of DOI:10.1016/S0040-4020(01)88382-4

The kinetics of the gas-phase thermal elimination of seven tert-butyl (hetero)arylcarboxylate esters (GCO2Bu^t) were measured over a temperature range <*> 50 K; the groups were the phenyl, 3- and 4-pyridyl, 2- and 3-furyl, and 2- and 3-thienyl moieties.The reactions were homogeneous and unimolecular with log A/s^-1 scaling 12.45 +/- 0.44, and with an entropy of activation uniformly negative and small.The kinetics obeyed a first-order rate equation, and at 600 K the rate coefficients (10³k/s^-1) of the esters, in the order given above, were: 54.1, 85.3, 120.5, 76.9, 43.2, 86.3 and 46.1, respectively.Hammett correlation of the present kinetic data with the literature ρ⁰ - values of the given substituents gave a reaction ρ constant compatible with the ethanoate molecular frame rather than with the carboxylate structure.The physical constants of five new t-butyl heteroarylcarboxylate esters are also described.