Carbon-hydrogen bonds radical reaction with

The reaction rate of molecular oxygen with alkyl radicals to form peroxy radicals (eq. 5) is much higher than the reaction rate of peroxy radicals with a hydrogen atom of the substrate (eq. 6). The rate of the latter depends on the dissociation energies (Table 1) and the steric accessibiUty of the various carbon—hydrogen bonds it is an important factor in determining oxidative stabiUty. 

A second process that has a central position in the analysis of the chemical properties of carbenes is their reaction with hydrocarbons. As is the case for alcohols, singlet and triplet carbenes react with hydrocarbons in distinctive ways. It has long been held that very electrophilic singlet carbenes can insert directly into carbon-hydrogen bonds (11) (Kirmse, 1971). On the other hand, triplet carbenes are believed to abstract hydrogen atoms to generate radicals that go on to combine and disproportionate in subsequent steps (12)... 

The feasibility of hydrogen abstraction at the peptidyl a-carbon hydrogen bond by 1,4-aryl diradicals has been determined by examining a model reaction, i.e. abstraction of deuterium from dideuterioglycine by aryl radicals. The results have biological implications for the reactivity of the enediyne anti-tumour antibiotics with proteins. The non-Arrhenius behaviour of hydrogen-abstraction reactions by radicals has been investigated. For a number of reactions studied the enthalpy of activation was found either to increase or to decrease as a function of temperature. 

Hydrocarbons undergo related reaction.s in the super-acid media, such as fluorosuiphuric acid and antimony pentachloride. It has been suggested that the initial one-electron processes during the electrochemical oxidation of alkanes in fluorosuiphuric acid involve a protonated carbon-hydrogen bond with formation of a carbon radical and release of two protons [15]. 

Reactivity ratios for all the combinations of butadiene, styrene, Tetralin, and cumene give consistent sets of reactivities for these hydrocarbons in the approximate ratios 30 14 5.5 1 at 50°C. These ratios are nearly independent of the alkyl-peroxy radical involved. Co-oxidations of Tetralin-Decalin mixtures show that steric effects can affect relative reactivities of hydrocarbons by a factor up to 2. Polar effects of similar magnitude may arise when hydrocarbons are cooxidized with other organic compounds. Many of the previously published reactivity ratios appear to be subject to considerable experimental errors. Large abnormalities in oxidation rates of hydrocarbon mixtures are expected with only a few hydrocarbons in which reaction is confined to tertiary carbon-hydrogen bonds. Several measures of relative reactivities of hydrocarbons in oxidations are compared. 

With the formation of free radicals having been initiated, these radicals react with oxygen (Reaction 3) to begin the propagation of the radical chains in forming a peroxy radical. The peroxy radical then attacks the 10-carbon-hydrogen bond to form the hydroperoxide radical (Reaction 4). [The possibility of such an intramolecular attack has been demonstrated in an aliphatic system where two reactive hydrogen atoms are located in the favorable 1,4-positions (9)]. 

Products which can be ascribed to the intermediate formation of radicals have long been observed in carbene reactions. In the gas phase these products could arise by homolytic decomposition of excited primary products before collisional deactivation rather than from radicals generated in the course of insertion. This is not so in solution. It is found that, in the thermal decomposition of diphenyldiazomethane (Bethell et al., 1965) or photolysis of diphenylketene (Nozaki et al., 1966) in toluene solution, the product of insertion of diphenylmethylene into the benzylic carbon-hydrogen bonds, 1,1,2-triphenylethane, is accompanied by substantial amounts of 1,1,2,2-tetraphenylethane and bibenzyl. This is a strong indication that discrete diphenylmethyl and benzyl radicals are formed, and, taken in conjunction with EPR (Section IIB) and other evidence (Etter et al., 1959) that diphenylmethylene is a ground-state triplet, would support the view that equation (20) is an adequate representation of triplet insertion. 

The phenolic initially gives up its labile hydrogen, which in turn reacts with the various radicals produced in chain reactions then the phenoxy radical becomes stabilized owing to its ability to form resonance structures. The resonance-stabilized forms of the phenoxy radical will not attack tertiary carbon—hydrogen bonds in the polypropylene chain but will react with other radicals such as a peroxide, resulting in the elimination of a second free radical. 

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