The mechanism of thermal eliminations. Part 19. Rate data for pyrolysis of S-aryl O-ethyl thiocarbonates: the relative abilities of oxygen and sulphur to transmit electronic effects

Abstract

Rates of thermal decomposition of a range of S-aryl O-ethyl thiocarbonates have been measured over a 50 °C range for each compound, between 722.6 and 671.4 K. The compounds are less reactive than the corresponding aryl ethyl carbonates by a factor of ca. 2.0 at 679 K, owing to the smaller –I effect of SAr relative to OAr. This provides further confirmation that the reactivity of esters towards thermal elimination parallels the extent of electron withdrawal by the group attached to the acyl carbon atom. The reaction gives a good Hammett correlation with σ° values, and ρ is 0.26 at 700 K, significantly greater than the value for the corresponding aryl ethyl carbonates. Thus sulphur is a better transmitter of electronic effects than is oxygen, and this transmission order, previously observed in studies in solution, is not a result of solvation phenomena. The ortho-methoxy substituent deactivates in thiocarbonate pyrolysis in contrast to its activation carbonates. This probably arises from through-space p_π→d_π bonding in the former compounds. An improved gold-plated stainless steel pyrolytic reactor is described.