Kinetics and Mechanism of Radical-Chain Oxidation of 1,2-Substituted Ethylene and 1,4-Substituted Butadiene-1,3

Abstract

A combination of microvolumetry, the rotating sector method, ESR, 1H NMR, and IR allowed to establish a detailed mechanism of liquid-phase oxidation of vinyl compounds X1CH=CHX2 and X1CH=CH–CH=CHX2 (X1 and X2—a polar substitute: С6Н5–, CO–, СOO–) initiated by azobisisobutyronitrile. A distinctive feature of the mechanism is the fact that the oxidation chain is carried out by a low-molecular hydroperoxide radical joining the π-bond. For nine compounds in the temperature range of 303–353 K, relative chain propagation and termination rate constants were measured (k2•k3−0.5). Absolute values of k2 were obtained for diphenylethylene (110 L·mol−1·s−1), ethyl ether of trans-phenyl-pentadiene acid (13 L·mol−1·s−1), and methyl ether of trans-phenyl-pentadiene acid (14.2 L·mol−1·s−1) at T = 323 K. For the same conditions, 10−8k3 were calculated for diphenylethylene (0.87 L·mol−1·s−1) and methyl ether of trans-phenyl-pentadiene acid (1.21 L·mol−1·s−1). A cyclic mechanism of the oxidation chain termination on introduced antioxidants (stable nitroxyl radicals of the piperidine series (>NO●) and the transition metal compounds (Men)) was established. The inhibition factor (f) showing how many reaction chains are terminated by the one particle of the antioxidant is equal to 102. The cyclic chain termination is caused by the following reactions: HO2● + >NO● → NOH + O2, HO2● + NOH → >NO● + H2O2 (for >NO●) and HO2● + Men → Men+1 + HO2●−, HO2● + Men+1 → Men + H+ + O2 (for Men).