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).

Kinetics of oxidation of benzyl alcohols with molecular oxygen catalyzed by N‐hydroxyphthalimide: Role of hydroperoxyl radicals

“…It is interesting to note that radicals HOO • play an essential role in the radical‐chain oxidation by molecular oxygen of 1,2‐substituted ethylenes where the chain‐propagating radical is not a polyperoxyl MOO • but a low molecular weight hydroperoxyl radical HOO • . The chain termination here occurs not as a recombination of the two MOO • radicals but as a disproportionation of the two HOO • radicals …”

“…The chain termination here occurs not as a recombination of the two MOO • radicals but as a disproportionation of the two HOO • radicals. 29 In many cases, reactions of HOO • radicals determine the efficiency of inhibition in radical-chain reactions. Hydroperoxyl plays an important role in vitamin E regeneration.…”

The Oxidative Polymerization of Vinyl Monomers in the Presence of N‐Hydroxyphthalimide

“…(v) Pliss et al . showed that radicals HOO • play an essential role in the reaction of radical‐chain oxidation with molecular oxygen of 1,2‐substituted ethylenes, and the chain propagating radical is not a polyperoxyl PO 2 • , but a low molecular weight hydroperoxyl radical HO 2 • .…”

“…(v) Pliss et al [56,57] showed that radicals HOO * play an essential role in the reaction of radical-chain oxidation with molecular oxygen of 1,2-substituted ethylenes, and the chain propagating radical is not a polyperoxyl PO 2 * , but a low molecular weight hydroperoxyl radical HO 2 * . The chain termination proceeds not as a recombination of the two radicals PO 2 * , but as a disproportionation of the radicals HOO * .…”

The Role of Solvation in the Kinetics and the Mechanism of Hydroperoxide Radicals Addition to π‐Bonds of 1,2‐Diphenylethylene and 1,4‐Diphenylbutadiene‐1,3

“…Hydroperoxide radicals () are carriers of liquid‐phase chain oxidation of organic molecules of different classes . During the oxidation of unsaturated compounds, depending on the substrate structure, the chain propagation takes place either via the addition reaction: + >CC< (M)→ HOOM • (≡M • ) or via the hydrogen atom abstraction from the αCHbond: + RH → HOOH + R • , or these routes are parallel . Along with liquid‐phase reactions, the possibility of formation is also considered in micro‐heterogeneous systems, particularly, the oxidation of methyl linoleate in micelles .…”