Hydrogen atoms, scavenging

Zheng W, Maurin M, Tarr MA (2005) Enhancement of sonochemical degradation of phenol using hydrogen atom scavengers. Ultrason Sonochem 12 313-317... 

For reasons which will become apparent in section 1.8.2, it is assumed that each electron-ion pair eventually produces one molecule of hydrogen. In an earlier section (p. 160) it was seen that in HC1 an additional 0.20+0.2 hydrogen atoms per ion pair should come from dissociative ionisation and charge transfer processes. In the absence of hydrogen atom scavengers they will undergo the reaction... 

DC1 solutions in cyclohexane have recently been studied (44, 52). Data similar to those from DI solutions have been obtained but can be more easily interpreted, at least in the low concentration region, since DC1 is a relatively poor hydrogen-atom scavenger. Here too, however, the total hydrogen yield increases upon adding DC1 [as noted earlier (II, 29)]. Allowing for a contribution of this excess to the HD yield, a value of aDci -— 10 can be estimated. 

Method D. Our fourth system uses ferf-butyl performate in a thiol free system to generate hydrogen atoms. Benzene is used as a hydrogen atom scavenger, and the absolute yield of hydrogen is measured when varying amounts of the hydrogen donor QH are added. 

To check the possible role of the hydrogen atoms in the first chemical reactions of the photosynthesis we have studied the action of a specific hydrogen atom scavenger the 2 propanol, on ... 

The 2 propanol acts also as an hydrogen atom scavenger ... 

Studies with simple radicals show that carbon-centered radicals react with phenols by abstracting a phenolic hydrogen (Scheme 5.14). The phenoxy radicals may then scavenge a further radical by C -C or C-O coupling or (in the case of hydroquinones) by loss of a hydrogen atom to give a quinone. The quinone may then react further (Section 5.4.4). Thus two or more propagating chains may be terminated for every mole of phenol.I9j... 

In contrast to phenols (Section 5,3.4), phenothiazine (39) is reported to be an excellent scavenger of both carbon-centered and oxygen-centered radicals by hydrogen atom transfer and is also used to stabilize monomers in storage.198... 

Irradiation of dilute aqueous solutions results in the interaction ofthe ionizing radiation with water molecules. The radiolysis of water produces hydrated electrons (eaq ", G = 2.8), hydrogen atoms (G = 0.6) and hydroxyl radicals (G = 2.8) which react with the molecules of the solutes. The use of special scavengers can convert one species to another, e.g. 

From chemical point of view, efficient free radical scavengers must contain substituents with the very weak C—H, O—H, or S—H bonds, from which reactive free radicals are able to abstract a hydrogen atom. It can be seen that the antioxidants discussed above (ascorbic acid, a-tocopherol, ubihydroquinones, glutathione, etc) fall under this category. However, many other compounds manifest free radical scavenging activity in in vitro and in vivo systems. 

The reaction of H atoms can be studied in acidic solution if the OH radicals are scavenged by t-butyl alcohol, in a very fast reaction, while hydrogen atoms react only very slowly with this alcohol (equations 5 and 6). The radical produced in equation 5 is relatively unreactive and does not interfere with the study of the reaction of H atoms with the solute. 

H" atom can both abstract hydrogen atoms and add to the double bonds. However it was found that the predominant reaction is the addition to the double bond. From the absorption of the cyclohexadienyl radical (formed by H abstraction) in acidic solution containing t-butanol (to scavenge the OH radicals) it was concluded14 that only 22% and 7% of the H atoms abstract hydrogen from 1,4- and 1,3-cyclohexadiene, respectively. 

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