Radical reactions hydrogen abstraction

Alvarez-Idaboy, J.R., N. Mora-Diez, R.J. Boyd, and A. Vivier-Bunge (2001b), On the importance of prereactive complexes in molecule-radical reactions Hydrogen abstraction from aldehydes by OH, J. Amer. Chem. Soc., 123, 2018-2024. 

It has been generally accepted that the thermal decomposition of paraffinic hydrocarbons proceeds via a free radical chain mechanism [2], In order to explain the different product distributions obtained in terms of experimental conditions (temperature, pressure), two mechanisms were proposed. The first one was by Kossiakoff and Rice [3], This R-K model comes from the studies of low molecular weight alkanes at high temperature (> 600 °C) and atmospheric pressure. In these conditions, the unimolecular reactions are favoured. The alkyl radicals undergo successive decomposition by [3-scission, the main primary products are methane, ethane and 1-alkenes [4], The second one was proposed by Fabuss, Smith and Satterfield [5]. It is adapted to low temperature (< 450 °C) but high pressure (> 100 bar). In this case, the bimolecular reactions are favoured (radical addition, hydrogen abstraction). Thus, an equimolar distribution ofn-alkanes and 1-alkenes is obtained. 

One side reaction commonly encountered in the reaction of alkyl-substituted polyenes with oxygen-centered radicals is hydrogen abstraction from the alkyl group in positions adjacent to the polyene jr-system. For reactions of the ferf-butyloxy radical, this reaction becomes so dominant that it can be used to form polyenyl radicals by hydrogen abstraction... 

After cyclization has taken place, the only reaction available to the cyclopentyl metiiyl radical is hydrogen abstraction. Thus it remains in solution until it reacts widi dibutyltin hydride to produce methylcyclopentane and a dibutyltin radical which continues die chain. 

Studies by electron-spin resonance spectroscopy showed that the reaction of hydroxy radicals with carbohydrates produces new radicals via hydrogen abstraction from a C-H group. When such carbohydrates as glucose are substrates, the H abstraction from a C-H bond is relatively nonselective, and all six possible radicals can be formed. The fate of these radicals is strongly influenced by the type of starting sugar and by the species present in the reaction medium. All of these aspects have been discussed in the review article by Sheldon et al.69... 

Hydrophobic protective systems include vitamin E, i.e. a-tocopherol which, as all chromanol compounds, is a free radical scavenger which yields a long-lived radical upon hydrogen abstraction, thereby interrupting the chain reaction [160]. This property is optimized in a-tocopherol (a-TH) which is a remarkable scavenger of peroxyl radicals in phospholipid membrane bilayers [161,162] ... 

The photochemical reactions of arenecarboxylic acid esters with alkenes has received recent attention by Cantrell. - For example, irradiation of 2,3-dimethyl-2-butene and methyl benzoate gave a mixture of alkoxyoxetane (56), carbonyl-alkene metathesis product (57) and ketone (58), resulting from alkoxy radical allylic hydrogen abstraction and radical recombination. Such alkoxyoxetane photoproducts are... 

Triplet diphenylcarbene, detected spectroscopically on excimer laser flash photolysis of diphenyldiazomethane, readily abstracts hydrogen from cyclohexane to give the CHPh2 radical.Analogous hydrogen abstraction also occurs from alcohols but at low temperatures and in a polycrystalline matrix in contrast to reaction in solution in which O-H bond insertion is preferred. In a similar fashion, irradiation of diphenyldiazomethane in polycrystalline (S)-butan-2-ol at — gave amongst other products an enantiomerically pure... 

The chain reaction is terminated by reactions that remove radicals that would otherwise produce more allylic radicals by hydrogen abstraction. Examples are the combination of two hydroperoxy radicals leading to nonradical products and molecular oxygen or reaction with a free-radical scavenger (antioxidant) generating a more stable radical. 

Atom Transfer (hydrogen abstraction) by LOO —> Free Radical Chain Reactions Hydrogen abstraction is the heart of the classic free radical chain reaction schemes (Figure 1). Peroxyl radicals initially formed at any site on a fatty acid pass the unpaired electron to adjacent lipid molecules by abstracting hydrogens from an allylic position or a hydroperoxide, and the process repeats itself indefinitely until the chain is intercepted. 

The abstraction of hydrogen from C(2) is typical of the general series of bicyclo[n.l.O]alkanes. Thus, the main reaction of these hydrocarbons with t-butoxy radicals is hydrogen abstraction from a position a to the cyclopropyl ring. This reaction is followed by a j -scission in which one of the cyclopropyl bonds is cleaved. Usually, the bond which is cleaved is the one which can assume an eclipsed conformation with the singly occupied orbital (SOMO). Hence, in the bicyclo[n.l.O]alkanes series, for n > 2, an outer bond is cleaved to give cycloalkenylmethyl radicals. However, for cases where n = 1 or 2, fission of the inner bond takes place yielding the cycloalkenyl radical (equation 87). ... 

The fact that irradiation of disilanes la - 3a did not produce detectable products other than the corresponding silyl radicals indicates that the radicals produced photochemically undergo clean recombination to produce the starting silane, as shown in Scheme 5 path (b). An exception is the photochemical reaction of silyl mercurial compounds 4c, which produce the hydrosilane 6 [Scheme 5, path (c)], while the corresponding dimer 4a [Scheme 5, path (b)] is not detected. Apparently, for radical 4b, hydrogen abstraction to produce silane 6 occurs significantly faster than its dimerization to 4a. 

Reaction (3) is documented in the literature [78] where it is said to take place when HBr is irradiated with 253.7 nm. In Burger s medium pressure mercury arc lamp, this wavelength is strongly represented. Reaction (3) regenerates bromine radicals, thus creating a steady state population of this species, so that the formation of furfural radicals by hydrogen abstraction can continue even after all of the bromofiirfural has disappeared. 

Peroxy radicals are obviously very important in the complex mechanisms of hydrocarbon oxidations. They propagate the reaction chains (eq. (2)), producing hydroperoxides. The selectivities of various alkylperoxy radicals in hydrogen abstractions have been reported to be relatively independent of the alkylperoxy radicals utilized [12, 15]. 

The materials we studied are non-aqueous dispersions of polymer particles. Colloidal stability of these particles in hydrocarbon solvents is conferred by a surface covering of a highly swollen polymer (the stabilizer) on a second polymer, insoluble in the medium (the core polymer), which comprises 90 % of the material (11). These particles are prepared by dispersion polymerization polymerization of a monomer soluble in the medium to yield an insoluble polymer, carried out in the presence of a soluble polymer which becomes the stabilizer. In the examples discussed here, the core polymer is formed by free radical polymerization. Hydrogen abstraction from the soluble polymer present in the reaction medium... 

Figure 8, Chain reaction photoinitiation. The efficiency of photoinitiation can be increased by chain reactions. Hydrogen abstraction by triplet excited benzophenone forms a THF free radical. Subsequent oxidation by the aryliodonium salt produces the THF cation capable of initiating polymerization, and a phenyl radical. Hydrogen abstraction by the phenyl radical produces the THF free radical completing the...

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