| Abstract: | The gas-phase eliminations of several tert-butyl esters, in a static system and in vessels seasoned with allyl bromide, have been studied in the temperature range of 171.5–280.1°C and the pressure range of 23–98 torr. The rate coefficients for the homogeneous unimolecular elimination of these esters are given by the following Arrhenius equations: for tert-butyl pivalate, log k1(s?1) = (13.44 ± 0.30) ? (169.1 ± 3.1) kJ · mol?1 (2.303RT)?1; for tert-butyl trichloroacetate, log k1(s?1) = (12.41 ± 0.08) ? (141.1 ± 0.7) kJ · mol?1 (2.303RT)?1; and for tert-butyl cyanoacetate log k1(s?1) = (11.31 ± 0.44) ? (137.8 ± 4.1) kJ · mol?1 (2.303RT)?1. The data of this work together with those reported in the literature yield a good linear relationship when plotting log k/k0 vs. σ* values (ρ* = 0.635, correlation coefficient r = 0.972, and intercept = 0.048 at 250°C). The positive ρ* value suggests that the movement of negative charge to the acyl carbon in the transition state is rate determining. The present results along with previous investigations ratify the generalization that electron-withdrawing substituents at the acyl side of ethyl, isopropyl, and tert-butyl esters enhance the elimination rates, while electron-releasing groups tend to reduce them. The negative nature of the acyl carbon and the polarity in the transition state increases slightly from primary to tertiary esters. |
