Hydrogen abstraction radical formation

Radiolysis of isobutyric acid at 195 K results in the formation of only one radical intermediate, the hydrogen abstraction radical III. The decarboxylation radical and the anion radical are both unstable at this temperature and react forming the abstraction radical and other products. The hydrogen which is abstracted is generally that which is attached to the carbon atom a to the carboxyl group. 

The presence of volatile nitrogen-containing products is indicative of main-chain scission. A plot of the total yield of these low molar mass products versus the yield of hydrogen abstraction radicals (which are formed from the primary radicals) is shown in Figure 6. The linear correlation with a slope close to one suggests that the formation of main-chain radicals is accompanied by the elimination of segments from the backbone polymer chain. 

Many authors assume that the initial reaction step in the dimerization is identical with that in the acrolein production, namely hydrogen abstraction and formation of an allylic intermediate. Dimerization is then supposed to occur because the ability to oxidize the allyl radical to acrolein is absent. 

Sites of Radical Initiation by Hydrogen Abstraction and Formation of Peroxyl Radicals... 

The proposed inactivation mechanism in Scheme 39 predicts that the normal reverse reaction, i.e., hydrogen abstraction from formate, may lead to a formate radical anion intermediate. Consistent with this idea, a isotope effect of... 

The stopped-flow kinetics of the anaerobic reactions were also studied. Reaction of linoleic acid with the yellow or purple enzymes results in (1) hydrogen abstraction, (2) formation of the radical L- (hence products of the anaerobic reactions), and (3) bleaching of the colored enzymes to Fe(II) species (fcg and k steps). Thus the rate of hydrogen abstraction (Ag and Ag) was measured as the rate of bleaching of the colored enzymes and found to be faster than Aj—formation of the yellow enzyme. This indicates that, in the presence of linoleic acid, the enzyme remains largely in the ferrous state (native enzyme) and that, unlike the situation in the aerobic reaction, hydrogen abstraction is not the rate-determining step in the anaerobic reaction. 

These results show that in the phenylation of thiazole with benzoyl peroxide two secondary reactions enter in competition the attack of thiazole by benzoyloxy radicals, leading to a mixture of thiazolyl benzoates, and the formation of dithiazolyle through attack of thiazole by the thiazolyl radicals resulting from hydrogen abstraction on the substrate and from the dimerization of these radicals. This last reaction is less important than in the case of thiophene but more important than in the case of pyridine (398). 

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. 

By using various trapping reagents, it has been deduced that the transannular fragmentation is rapidly reversible. The cyclization of the fragmented radical C is less favorable, and it is trapped at rates which exceed that for recyclization under most circumstances. " Radicals derived from ethers and acetals by hydrogen abstraction are subject to fragmentation, with formation of a ketone or ester, respectively. 

In most cases the carbon radical formed in the hydrogen abstraction step 2 will react with the radical R formed in the homolysis of the X—R bond. However, a cage reaction does not seem to be involved in this step. This has been established in the nitrite photolysis and probably applies to hypohalites as well. In the lead tetraacetate reaction, the steps following the oxyradical formation leading to tetrahydrofuran derivatives are less clear. 

The alkoxycarbonyloxy radicals show little tendency to abstract hydrogen.188,431 For example, in the reaction of isopropoxycarbonyloxy radicals with MMA, hydrogen abstraction, while observed, is a minor pathway (<1%). When isopropoxycarbonyloxy radicals abstract hydrogen, isopropanol is the expected byproduct since the intermediate acid undergoes facile decarboxylation. Formation of isopropanol is not evidence for the involvement of isopropoxy radicals (Scheme 3.80). 

The microstrueture of PVC has been the subject of numerous studies (Sections 4.3.1.2 and 6.2.6.3).214 Starnes el n/.l6S determined the long chain branch points by NMR studies on PE formed by Bu,SnlI reduction of PVC. They concluded that the probable mechanism for the formation of these branches involved transfer to polymer that occurred by hydrogen abstraction of a backbone methine by the propagating radical (Scheme 6.32),... 

The formation of dimethyl sulfide, dimethyl sulfone, and methane (by H-abstraction) observed in these photolyses is thus accounted for. Hydrogen abstraction by the methylsulfinyl radical affords methanesulfenic acid, CH3SOH, a very reactive molecule, which rapidly undergoes a series of secondary reactions to produce the methanesulfonic acid, methyl methanethiolsulfonate (CH3S02SCH3), and dimethyl disulfide which were also observed during these photolyses. 

Alternatively, two radicals can interact via hydrogen abstraction, leading to the formation of two reaction products, one of which is saturated and one of which is unsaturated. This is known as disporportionation (Reaction 2.6). 

TTx represents the hydrophobicity of the substituents at position 10. Its positive coefficient (+0.75) suggests that the presence of highly hydrophobic substituents at position 10 increases the activity. The outlier (X = OH) is much more active than expected by 11 times the standard deviation. This may be due to the formation of a phenoxyl radical that interacts with DNA [48]. The other derivative (X = NH2) is also considered as an outher due to being much more active than expected by 14 times the standard deviation. This anomalous behavior may be attributed to its nature as an aniline. This could result in hydrogen abstraction, or involve microsomal N-oxidation [48,49]. 

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