The mechanism of the gas-phase pyrolysis of esters. Part IV. Effects of substituents at the β-carbon atom
Rates of gas-phase thermal decomposition of some 2-substituted ethyl acetates and 3-substituted propyl acetates into acetic acid and substituted alkenes have been measured between 368.7 and 429.2 °C. The data for 2-aryl-ethyl a acetates give a Hammett correlation with ρ+0.2 at 650 K, which taken along with previous data for the effect of α-aryl substituents (ρ–0.63 at 625 K) confirm that electron return to the β-carbon does not compensate for the electron withdrawal from the α-carbon during the transition state for the reaction so that the process is not fully concerted. These data, and comparison of rates of elimination of 2-substituted ethyl acetates with those of the corresponding 3-substituted propyl acetates provides the first unified and consistent interpretation of the effects of β-substituents on the rates of pyrolysis of esters. The data show that the conjugative effects of 3-substituents play a major role in determining the overall rate of elimination for compounds with β-substituents, and that the anomalous activating effect of β-alkyl substituents is not due to the stabilising effect upon the incipient α-carbonium ion as reported in the literature, but rather to steric acceleration.