GAS-PHASE PYROLYTIC REACTIONS .3. HAMMETT-RHO CONSTANTS OF GAS-PHASE ELIMINATIONS OF ALKYL ARYLCARBOXYLATE AND ETHANOATE ESTERS AND THEIR CORRELATION WITH SUBSTRATE MOLECULAR FRAMEWORK

The kinetics of the gas-phase thermal elimination of seven tert-butyl (hetero)arylcarboxylate esters (GCO2But) were measured over a temperature range = 50 K; the groups were the phenyl, 3- and 4-pyridyl, 2- and 3-fnryl, and 2- and 3-thienyl moieties. The reactions were homogeneous and unimolecular with log A/s-1scaling 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 (l03k/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 σ0 values of the given substituents gave a reaction ρ{variant} 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. © 1990.