Activating effects of unsaturated and cyclopropyl groups in gas phase pyrolysis of acetates. Paper XVIII

The activating effects of a number of unsaturated groups and a cyclopropyl group have been evaluated in a solvent free system by determining the absolute rate constants, and energies and entropies of activation in the vapor phase pyrolysis of secondary and tertiary esters of the type RC(R′CH 3 ) OAc where R′ = H or CH 3 and R = c ‐Pr, i ‐Pr, CH 3 , CH 2 CH, CH 2 CHCH 2 , C 6 H 5 ; the cyclopropyl showed only a moderate activating effect. The results are in contrast to the very significant activating effect of a cyclopropyl group in solvolysis of cyclopropylcarbinyl derivatives. Apparently marked activation by this group occurs only when a highly developed positive center forms adjacent to it. The lack of marked activation by the cyclopropyl group supports a mechanism for ester pyrolysis which involves a modest, but detectable, charge separation in the transition state [2] but questions a mechanism in which an intimate ion‐pair was proposed [3].