GAS-PHASE PYROLYTIC REACTIONS .4. ARRHENIUS PARAMETERS AND HAMMETT-SIGMA-0-CONSTANTS IN GAS-PHASE ELIMINATIONS OF ALKYL PYRIDYLCARBOXYLATE, FURYLCARBOXYLATE, AND THIENYLCARBOXYLATE ESTERS

The first-order rate coefficients (105K/s-1) of the gas-phase pyrolytic reactions of seven isopropyl (hetero)arylcarboxylate esters were calculated for 600 K to be: 75.34 for benzoate, and 100.0, 70.46, 94.03, 65.58, 100.9 and 120.2 for 2-thienyl-, 3-thienyl-, 2-furyl, 3-furyl, 3-pyridyl- and 4-pyridylcarboxylate, respectively. The corresponding Hammett replacement σ0 substituent constants of the heterocyclic groups are: 0.53, -0.13, 0.42, -0.26, 0.55 and 0.88, respectively. The reported σo constants are in agreement with other gas-phase and solution data, and are amenable to rationalization in terms of normal electronic and structural effects. Further, the physical constants of four new isopropyl heteroarylcarboxylate esters are described. © 1990.