Abstraction reaction with organic molecules

Photochemical reaction with organic molecules within the tumour cells, where hydrogen abstraction occurs. This initiates a number of radical reactions, resulting in destruction of the tumour. 

Although OH behaves mechanistically like H in its reactions with organic molecules, it is less selective and more reactive than H in H-abstraction reactions because the formation of the H—OH bond is 57 kJ mol more exothermic than that of H—H. Examples of the reactivities of H , OH, and 0 are listed in Table 4. 

In its reactions with organic molecules, OH behaves as an electrophile whereas 0 is a nucleophile. Thus, like H (see above), OH readily adds to double bonds but 0 does not however, both forms of the radical abstract H from C-H bonds. In the case of an aromatic molecule carrying an aliphatic side chain, OH adds preferentially to the aromatic ring and 0 abstracts H from the side chain this can result in a change of reaction mechanism when the pH is raised so that 0 replaces OH as the oxidant. 

An important property of EPR spectroscopy is the detailed information provided by the structure of the spectra so that there is higher degree of certainty of correctly identifying the radicals than is the case with optical spectroscopy. Thus, chemically similar radicals that would be expected to have similar UV absorption spectra have completely different EPR spectra. For example, in its reactions with organic molecules, OH can attack at a number of different sites, either to abstract a hydrogen atom from a saturated molecule (see Table 5) or to add to an aromatic ring. Each of the product radicals has a distinctive EPR spectrum, and the relative efficiency with which reaction occurs at each site on the target molecule can be determined from the intensities of the spectra of the various radical products. 

In its reactions with organic molecules, OH is an electrophile whereas O " is a nucleophile. Forexample, in their reaction with aromatic compounds containing an aliphatic side chain, OH adds preferentially to the aromatic ring but O" abstracts H from the side chain ... 

HO2 and 02 have characteristic absorption spectra with s ax 140 mol at 225 nm [83] and Smav = 189 mol at 245 nm [85], respectively, which are sufficiently intense to permit their reactions to be followed by direct observation in pulse radiolysis experiments. Both radicals are relatively unreactive with organic molecules [83], abstracting only weakly bonded hydrogen atoms in, for example, ascorbic acid, cysteine, and hydroquinone. Oj undergoes reversible electron transfer in its reaction with quinones (Q), which was used to establish its reduction potential [86] ... 

The 02 ion appears to play an important role in a number of photooxidation reactions (see Section VI,C) for example, the photo-oxidation of alkenes over TiOz. However, it seems likely that OJ is not, in many cases, active in the oxidation step but further conversion occurs to give a mononuclear species, not detected directly, which then oxidizes the adsorbed hydrocarbons. Photo-oxidation of lattice oxygen in the M=0 systems (e.g., V2Os supported on PVG) gives rise to an excited charge transfer state such as V4 + -0 . This excited state can react as O- either by addition to a reactant molecule or by an abstraction reaction (see Section V of Ref. /). In the presence of oxygen, 03 is formed which then reacts further with organic molecules. 

Difluorine is an extremely active reagent and reacts with organic molecules at low (200 and lower) temperatures. Such a high activity of difluorine is due to the very high BDE of the formed hydrogen fluoride molecule (-DF H = 570 kJ mol ) and the relatively low BDE in the difluorine molecule (DF F = 158.7 kJ mol-1). Due to this great difference in the BDE of the reactants and the products, bimolecular reactions of hydrogen atom abstraction... 

Laird, D. A. 2004. Reactions of organic molecules with smectite surfaces in aqueous systems. Acta Mineralogica-Petrographics, Abstract Series 4, p. 64. University of Szeged, Hungary. 

Since it is unanimously agreed that MTBE is eUminated via OH radicals, many studies have been performed in order to elucidate the mechanistic pathway of the conversion. OH radicals react with organic molecules by the abstraction of hydrogen from C - H or O - H bonds, via hydroxyl group addition to unsaturated carbon bonds or the interaction with N-, P-, and S-containing bonds [32]. Since in the MTBE molecule no unsaturated carbon bonds and no N-, P-, or S-atoms are present the initial reaction has to concentrate on the abstraction of H atoms or the cleavage of the C - 0 bond. 

In its reactions with organic compounds the hydrogen atom generally abstracts H from saturated molecules and adds to the centers of unsaturated the fate of the organic radicals is well known (Chapter 5.3.3) ... 

The autoxidation of aldehydes, and of other organic compounds, may be lessened considerably by very careful purification—removal of existing peroxides, trace metal ions, etc.—but much more readily and effectively by the addition of suitable radical inhibitors, referred to in this context as anti-oxidants. The best of these are phenols and aromatic amines which have a readily abstractable H atom, the resultant radical is of relatively low reactivity, being able to act as a good chain terminator (by reaction with another radical) but only as a poor initiator (by reaction with a new substrate molecule). 

Hirschler and Hudson (36/6), however, favor the opinion that Bronsted sites are exclusively responsible for the activity of silica-alumina. In studying the adsorption of perylene and of triphenylmethane, they concluded that carbonium ions are not formed by a hydride abstraction mechanism as claimed by Leftin (362). Instead, triphenylmethane is oxidized by chemisorbed oxygen to triphenylcarbinol in a photo-catalyzed reaction, followed by reaction with a Bronsted acid giving water and a triphenylmethyl carbonium ion. After treatment with anhydrous ammonia, the organic compound was recovered by extraction as triphenylcarbinol. About thirteen molecules of ammonia per assumed Lewis site were required to poison the chemisorption of trityl ions. The authors explain the selective inhibition of certain catalyzed reactions by alkali by assuming that only certain of the acidic protons will ion-exchange with alkali ions. 

Abstract Palladium-catalyzed oxidation reactions are among the most diverse methods available for the selective oxidation of organic molecules, and benzoquinone is one of the most widely used terminal oxidants for these reactions. Over the past decade, however, numerous reactions have been reported that utilize molecular oxygen as the sole oxidant. This chapter outlines the fundamental reactivity of benzoquinone and molecular oxygen with palladium(O) and their catalyst reoxidation mechanisms. The chemical similarities... 

---