Kinetics of the Free-Radical Nonbranched-Chain Addition

Abstract

The aim of this study was the conclusion of simple kinetic equations to describe ab initio initiated nonbranched-chain processes of the saturated free-radical addition to the double bonds of unsaturated molecules in the binary reaction systems of saturated and unsaturated components. In the processes of this kind the formation rate of the molecular addition products (1:1 adducts) as a function of concentration of the unsaturated component has a maximum. Five reaction schemes are suggested for this addition processes. The proposed schemes include the reaction competing with chain propagation reactions through a reactive free radical. The chain evolution stage in these schemes involves three or four types of free radicals. One of them is relatively low-reactive and inhibits the chain process by shortening of the kinetic chain length. Based on the suggested schemes, nine rate equations (containing one to three parameters to be determined directly) are deduced using quasi-steady-state treatment. These equations provide good fits for the nonmonotonic (peaking) dependences of the formation rates of the molecular products (1:1 adducts) on the concentration of the unsaturated component in binary systems consisting of a saturated component (hydrocarbon, alcohol, etc.) and an unsaturated component (alkene, allyl alcohol, formaldehyde, or dioxygen). The unsaturated compound in these systems is both a reactant and an autoinhibitor generating low-reactive free radicals. A similar kinetic description is applicable to the nonbranched-chain process of the free-radical hydrogen oxidation, in which 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.

Free-Radical Nonbranched-Chain Oxidation of Hydrogen

“…1). In such reaction systems consisting of saturated and unsaturated components [51,52], the unsaturated compound (in this case the O 2 ) is both a reactant and an autoinhibitor, specifically, a source of low-reactive free 8 Note that in the case of similar nonbranched-chain (i.e., "slow") oxidation of RH hydrocarbons, the corresponding reactive [50]. The only difference between the kinetic model of oxidation and the kinetic model of the chain addition of 1-hydroxyalkyl radicals to the free (unsolvated) form of formaldehyde in nonmethanolic alcohol-formaldehyde systems [1,9,10] is that in the former does not include the formation of the molecular 1:1 adduct via reaction 4. radicals (in this case the  4 HO radicals) shortening kinetic chains.…”