Hydrogen atom abstraction from water

Negligible photoreaction was observed for p,p -dichlorobenzophenone (DCB), a DDT oxidation product, in air-saturated, distilled water (half-fife >15 h at 313 nm). Nevertheless, this halocarbon photoreacted (313 nm) with half-lives corrected for light attenuation of about 3 h in a filtered natural-water sample and a solution of Contech fulvic acid (Table II). The greater than four- to fivefold enhancement in photoreaction rate in this case probably results from hydrogen atom abstraction from the natural organic matter by the DCB in its excited triplet state (eq 14). 

Anilines are virtually always present as by-products, however, in some cases, they are also intermediates in the synthesis of carbamates and ureas. This last possibility will be discussed in more detail for the individual catalytic systems. When the aniline is a by-product, the necessary hydrogen atoms can originate from small amounts of water or H2 present as impurities in the solvent and/or the CO gas, but can also be produced by alcohol dehydrogenation (when an alcohol is present) or, to a minor extent, by direct hydrogen-atom abstraction from the solvent or from the nitro compound itself. Among primary alcohols, methanol is the less prone to be dehydrogenated [8]. 

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). ... 

Some conclusions on the reaction mechanism may be drawn from the rate constants obtained. It was shown for hydroxyl reactions with saturated compounds (propane, for example) that the main reaction of OH was the hydrogen atom abstraction in the formation of water. This is an accepted point of view. However, another route is possible for reactions with unsaturated hydrocarbons, i.e., addition at the double bond. This is the case for the H atom with saturated compounds H reacts by abstraction, and with unsaturated ones by addition. 

Figure 2.24. Overview of the membrane-associated cytochrome P450 system. Hydrogen is abstracted from NADPH and used to reduce one atom of molecular oxygen to water the second oxygen atom reacts with the substrate, often forming a hydroxyl group.

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