Hydroxyl radicals, hydrogen abstraction

The hydroxyl radicals may abstract hydrogen which gives a radical on the substrate (P-) that can initiate polymerization by adding a vinyl monomer (M) ... 

The hydroxyl radicals may abstract hydrogen from the substrate (which initiates grafting) (16) or add to a vinyl monomer (which gives homopolymer). The decomposition rate of persulfate ions is enhanced by the presence of a low-molecular weight alcohol. A high-molecular weight alcohol like cellulose can react in the same way. 

Atmospheric reactions of MTBE are often initiated by hydroxyl radicals that abstract hydrogen from the methyl groups to form water and organic rascals. There are various kinds of models and mechanisms postulated. The following are two possible initial reactions. For hydrogen radical abstraction at the methoxy end ... 

Products of degradation alkoxy and hydroxy radicals, hydrogen abstraction, formation of carbonyls, and hydroxyl groups ... 

B. S. Jursic, Chem. Phys. Lett., 256,603 (1996). Density Functional Theory Study of Radical Hydrogen Abstraction with Hydrogen and Hydroxyl Radicals. 

Hydroxyl radicals may abstract hydrogen from tertiary carbons and produce polymer alkyl radicals and water ... 

Many hexaaquametal ions, e.g., of Fe " ", Mn, Cu, react with hydroxyl radicals by abstracting a hydrogen atom of the coordinated water, this being the slowest and therefore the rate-determining step ... 

The complete reaction scheme is shown in Fig. 5.1 for the photolytic dissociation, highlighting how the initial spin state of the hydroxyl radical is trapped in the 2-propanolyl radicals. Hydrogen abstraction of the 2-propanol mainly occurs at the a-position, however, the reaction is non-selective and multiple secondary radicals are produced [8] with the yields of the products indicated in Fig. 5.2. 

As a class of compounds, the two main toxicity concerns for nitriles are acute lethality and osteolathyrsm. A comprehensive review of the toxicity of nitriles, including detailed discussion of biochemical mechanisms of toxicity and stmcture-activity relationships, is available (12). Nitriles vary broadly in their abiUty to cause acute lethaUty and subde differences in stmcture can greatly affect toxic potency. The biochemical basis of their acute toxicity is related to their metaboHsm in the body. Following exposure and absorption, nitriles are metabolized by cytochrome p450 enzymes in the Hver. The metaboHsm involves initial hydrogen abstraction resulting in the formation of a carbon radical, followed by hydroxylation of the carbon radical. MetaboHsm at the carbon atom adjacent (alpha) to the cyano group would yield a cyanohydrin metaboHte, which decomposes readily in the body to produce cyanide. Hydroxylation at other carbon positions in the nitrile does not result in cyanide release. 

The effect substitution on the phenolic ring has on activity has been the subject of several studies (11—13). Hindering the phenolic hydroxyl group with at least one bulky alkyl group ia the ortho position appears necessary for high antioxidant activity. Neatly all commercial antioxidants are hindered ia this manner. Steric hindrance decreases the ability of a phenoxyl radical to abstract a hydrogen atom from the substrate and thus produces an alkyl radical (14) capable of initiating oxidation (eq. 18). 

The attack on the aromatic nucleus by hydroxyl radicals is probably analogous to that by phenyl and methyl radicals, Eq. (34a,b). Evidence that the first step is the addition of hydroxyl radical to benzene, rather than abstraction of a hydrogen atom, has recently been adduced from a study of the radiolysis of water-benzene mixtures. The familiar addition complex may undergo two reactions to form the phenolic and dimeric products respectively, Eq. (34a,b). Alternative mechanisms for the formation of the dimer have been formulated, but in view of the lack of experimental evidence for any of the mechanisms further discussion of this problem is not justified. 

Hydroxy radical and sulfate radical anion, though they may sometimes give rise to similar products, show quite different selectivity in their reactions with unsaturated substrates. In particular, the sulfate radical anion has a somewhat lower propensity for hydrogen abstraction than the hydroxyl radical. For example, the sulfate radical anion shows little tendency to abstract hydrogen from mcthacrylic acid.232... 

NMHC. A large number of hydrocarbons are present in petroleum deposits, and their release during refining or use of fuels and solvents, or during the combustion of fuels, results in the presence of more than a hundred different hydrocarbons in polluted air (43,44). These unnatural hydrocarbons join the natural terpenes such as isoprene and the pinenes in their reactions with tropospheric hydroxyl radical. In saturated hydrocarbons (containing all single carbon-carbon bonds) abstraction of a hydrogen (e,g, R4) is the sole tropospheric reaction, but in unsaturated hydrocarbons HO-addition to a carbon-carbon double bond is usually the dominant reaction pathway. 

The major reactions carried out by hydroxyl and nitrate radicals may conveniently be represented for a primary alkane RH or a secondary alkane RjCH. In both, hydrogen abstraction is the initiating reaction. 

The stability of perchlorofluoroalkanes is due to the absence of hydrogen atoms that may be abstracted by reaction with hydroxyl radicals. Attention has therefore been directed to chlorofluo-roalkanes containing at least one hydrogen atom (Hayman and Derwent 1997). Considerable effort has also been directed to the reactions of chloroalkanes and chloroalkenes, and this deserves a rather more detailed examination in the light of interest in the products that are formed. 

Radicals can react with bases via hydrogen atom abstraction or, more commonly, by addition to the pi bonds in the heterocyclic nucleobases (Scheme 8.1). These reactions have been extensively studied in the context of hydroxyl radical (HO ), which is generated by y-radiolysis of water. When DNA is exposed to the hydroxyl radical, approximately 80% of the reactions occur at the bases. Many base damage products arising from the reaction hydroxyl radical with DNA have been characterized (Fig. 8.2). ... 

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