Hydrogen atom abstraction temperature elevations

Oxidation of C—bonds by copper ion catalyzed reaction with an organic peroxy ester (the Kha-rasch-Sosnovsky reaction) was at one time very popular for allylic oxidation and has been thoroughly reviewed. The reaction is usually carried out by dropwise addition of peroxy ester (commonly /-butyl peracetate or /-butyl perbenzoate) to a stirred mixture of substrate and copper salt (0.1 mol % commonly copper(I) chloride or bromide) in an inert solvent at mildly elevated temperature (60-120 C). The mechanism involves three steps (i) generation of an alkoxy radical (ii) hydrogen atom abstraction and (iii) radical oxidation and reaction with carboxylate anion (Scheme 11). 

The formation of the backbone radical site depends on the nature of the polyolefin. The presence of tertiary carbon atoms as in polypropylene and branch points in low-density polyethylene, LDPE, would be expected to provide the preferred site for hydrogen-atom abstraction. However, it has been noted (Russell, 2002) that the difference between the reactivity of tertiary and secondary carbon atoms decreases at elevated temperatures. 

The peroxide decomposes at elevated temperatures to give free radicals, which then abstract a hydrogen atom from the methyl group. The radicals formed then combine to form a hydrocarbon linkage. Results obtained by reacting model systems with benzoyl peroxide and analysing the reaction products are consistent with this type of mechanism. ... 

The traditional chain oxidation with chain propagation via the reaction RO/ + RH occurs at a sufficiently elevated temperature when chain propagation is more rapid than chain termination (see earlier discussion). The main molecular product of this reaction is hydroperoxide. When tertiary peroxyl radicals react more rapidly in the reaction R02 + R02 with formation of alkoxyl radicals than in the reaction R02 + RH, the mechanism of oxidation changes. Alkoxyl radicals are very reactive. They react with parent hydrocarbon and alcohols formed as primary products of hydrocarbon chain oxidation. As we see, alkoxyl radicals decompose with production of carbonyl compounds. The activation energy of their decomposition is higher than the reaction with hydrocarbons (see earlier discussion). As a result, heating of the system leads to conditions when the alkoxyl radical decomposition occurs more rapidly than the abstraction of the hydrogen atom from the hydrocarbon. The new chain mechanism of the hydrocarbon oxidation occurs under such conditions, with chain... 

The presence of hydrocarbon impurities has been shown to affect the oxidation of carbon monoxide [22] and the decomposition of carbon dioxide [23]. It has been reported that the dissociations of ethane [24] and butane [25] at the elevated temperatures and typical densities of shock tube experiments are in the low pressure region with activation energies that are much less than their respective high pressure limit values. The reaction of p.p.m. levels of hydrogen atoms with the molecule under investigation can result in a low apparent energy for dissociation due to the increased importance of abstraction steps. 

Along with reactions without the participation of the radical centre, ARs react as typical radicals. At elevated temperatures, they abstract hydrogen atoms, chlorine, bromine and other elements. There are examples of sufficiently reactive ARs in H-atom abstraction at standard temperatures. The benzotriazole-N-oxyl (BTNO) generated by the oxidation of 1-hydroxybenzotriazole (HBT) with a Ce" salt in acetonitrile spontaneously decays with a first-order rate constant of 6.3 x 10" s" at room temperature [31]. The decay of this aminoxyl is strongly accelerated in the presence of H-donor substrates such as alkylarenes, benzyl and allyl alcohols ... 

The y-radiolysis of PMMA at room temperature in an atmosphere of NO as well as photolysis leads to the formation of acylalkylaminoxyl radicals. The evacuation of samples at elevated temperatures gives rise to the appearance of the signal of iminoxyl radicals in the ESR spectrum. As distinct from photolysis, y-radiolysis can stimulate hydrogen-atom detachment from the C-H bonds of macromolecules and, consequently, the formation of primary and secondary macromolecular nitroso compounds takes place in an atmosphere of NO. Such nitroso compounds are rapidly isomerised into oximes [63]. The abstraction of hydrogen atoms from the OH groups of oximes by active free radicals results in the formation of iminoxyl radicals [64]. This viewpoint is confirmed by the observation of ESR spectra of iminoxyl radicals in y-irradiated PMMA and AC in the presence of NO. 

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