Abstract
New reaction scheme are suggested for the initiated nonbranched-chain addition of free radicals to the multiple bond of the molecular oxygen. The scheme includes the reaction competing with chain propagation reactions through a reactive free radical. The chain evolution stage in this scheme involves a few of free radicals, one of which (tetraoxyl) is relatively low-reactive and inhibits the chain process by shortening of the kinetic chain length. Based on the proposed scheme rate equations (containing one to three parameters to be determined directly) are deduced using quasi-steady-state treatment. The kinetic description with use the obtained rate equations is applied to the ϒ-induced nonbranched-chain processes of the free-radical oxidation of liquid o-xylene at 373 K and hydrogen dissolved in water containing different amounts of oxygen at 296 K. In these processes the oxygen with the increase of its concentration begins to act as an oxidation autoingibitor (or an antioxidant). The energetics of the key radical-molecule reactions is considered.
Highlights
The kinetics of inhibition for nonbranched-chain processes of saturated free-radical addition to the C=C and C=O double bonds of alkene and formaldehyde molecules, respectively, by low-reactive free radicals that can experience delocalization of the unpaired pelectron was first considered in [1]
In the present work the kinetics of inhibition by low-reactive tetraoxyl free radicals is considered for nonbranched-chain processes of the addition of a free radical or an atom to one of the two multiply bonded atoms of the oxygen molecule yielding a peroxyl free radical and initiating oxidation, which is the basic process of chemical evolution
A number of experimental findings concerning the autoinhibiting effect of an increasing oxygen concentration at modest temperatures on hydrogen oxidation both in the gas phase [12, 35, 36] (Figure 2) and in the liquid phase [34] (Figure 1, curve 2), considered in our earlier works [13, 27, 28, 37], can be explained in terms of the competition kinetics of free radical addition [14, 38]
Summary
The kinetics of inhibition for nonbranched-chain processes of saturated free-radical addition to the C=C and C=O double bonds of alkene and formaldehyde molecules, respectively, by low-reactive free radicals that can experience delocalization of the unpaired pelectron was first considered in [1]. The only reaction that can compete with these two reactions at the chain evolution stage is the addition of the peroxyl radical to the oxygen molecule (provided that the oxygen concentration is sufficiently high). This reaction yields the secondary tetraoxyl 1:2 adduct radical, which is the heaviest and the largest among the reactants.
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