Hydrogen Atom Abstraction Reaction

Hydrogen Abstraction Photoexcited ketone intermolecular hydrogen atom abstraction reactions are an interesting area of research becanse of their importance in organic chemistry and dne to the complex reaction mechanisms that may be possible for these kinds of reactions. Time resolved absorption spectroscopy has typically been nsed to follow the kinetics of these reactions but these experiments do not reveal mnch abont the strnctnre of the reactive intermediates. " Time resolved resonance Raman spectroscopy can be used to examine the structure and properties of the reactive intermediates associated with these reactions. Here, we will briefly describe TR experiments reported by Balakrishnan and Umapathy to study hydrogen atom abstraction reactions in the fluoranil/isopropanol system as an example. 

Even if atomic oxygen-carbon cluster reactions are rapid, as is assumed in the new neutral-neutral model, the synthetic power of this model can be recovered if exothermic hydrogen atom abstraction reactions of the sort,... 

It has been proposed that this reaction intermediate could decompose to produce HCN and CH3 [55], Chemiluminescence from alkanes can be greatly enhanced by addition of HC1. The proposed explanation is that energy transfer from active nitrogen dissociates HC1 to produce chlorine atoms, which have rapid hydrogen-atom abstraction reactions with alkanes,... 

This conclusion is partly true because superoxide is unable to abstract hydrogen atom even from the most active bisallylic positions of unsaturated compounds, while perhydroxyl radical abstracts H atom from linoleic, linolenic, and arachidonic fatty acids with the rate constants of 1-3 x 1031 mol-1 s-1 [24], However, the superoxide damaging activity does not originate from hydrogen atom abstraction reactions but from one-electron reduction processes, leading to the formation of hydroxyl radicals, peroxynitrite, etc, and in these reactions perhydroxyl cannot compete with superoxide. 

Antioxidant activity of flavonoids has already been shown about 40 years ago [90,91]. (Early data on antioxidant flavonoid activity are cited in Ref. [92].) Flavonoids are polyphenols, and therefore, their antioxidant activity depends on the reactivity of hydroxyl substituents in hydrogen atom abstraction reactions. As in the case of vitamins E and C, the most studied (and most important) reactions are the reactions with peroxyl radicals [14], hydroxyl radicals [15], and superoxide [16]. 

Nature has designed the active sites in the enzymes GO and glyoxal oxidase in order to perform a hydrogen-atom abstraction reaction from the a-carbon atom of an O-bound alcoholate (or aldehyde) in the rds. The essence of this chemistry is depicted in Fig. 8. 

In kinetic studies, the abstraction of F from CF3CH2OH by ArN2+BF4 was shown to be an ionic process, whereas abstraction of an a-H atom from CH3CH2OH was determined to be a radical process. In other kinetic work, the expansion of cyclopentanones has been used to measure the kinetics of hydrogen-atom abstraction reactions for a range of hydrogen donors in benzene including (Me0)2P(0)H( 1.2 x... 

High-level computational methods are limited, for obvious reasons, to very simple systems. In the previous section we showed the contribution of the theory for a better imderstanding of the entropic and enthalpic factors that influence the reactions of hydrogen atom with the simplest series of silanes Me4 SiH , where n = 1-3. Calculated energy barriers for the forward and reverse hydrogen atom abstraction reactions of Me, Et, i-Pr and t-Bu radicals with Me4- SiH , where n = 0-3, and (H3Si)3SiH have been obtained at... 

Camphorquinone (12), which has been studied in several solvents, gives typical hydrogen atom abstraction reactions. It undergoes photoreduction in isopropyl alcohol39,71,72 and forms mixtures of solvent addition and reduction products when irradiated in methanol,39,71 aldehydes,73 and xylene.23 No transformation is observed in benzene or carbon tetrachloride.74... 

Figure 10.9. Orbitals for a hydrogen atom abstraction reaction. The middle is the orbital diagram for the transition state for H transfer.

These redox chain reactions, which cycle iron(II) and iron(III), have advantages over methods that use stoichiometric quantities of oxidants because the hydroxymethyl radical is also a good reductant and, at high oxidant concentrations, it may be oxidized more rapidly than it adds to (72). The disadvantage of this type of reaction is that the initial radical is generated by a relatively non-selective hydrogen atom abstraction reaction. To be efficient, the H-donor must be used in large excess it is often a cosolvent. Nonetheless, this is a very practical method to prepare hydroxyalkylated and acylated heteroaromatic and related derivatives. 

Gilbert BC, Lindsay Smith JR, Taylor P, Ward S, Withwood AC (1999) The interplay of electronic, steric and stereoelectronic effects in hydrogen-atom abstraction reactions of S04, revealed by EPR spectroscopy. J Chem Soc Perkin Trans 2 1631-1637 Flasegawa K, Neta P (1978) Rate constants and mechanisms of reaction of Cl2 radicals. J Phys Chem 82 856-859... 

LMCT states. Unlike bimolecular, outer-sphere, hydrogen-atom abstraction reactions, the formation of zwitterionic products is a typical innercomplex process. 

Values of k+ are known from gas phase studies for a variety of unsaturated compounds91 and with these non-polar solvents it may be assumed that the values will be the same in the liquid phase. Hence it has been possible to determine values of the rate coefficients for a wide series of hydrogen atom abstraction reactions in solution. On the basis of these, it has been possible to determine values of kA for other scavengers92. Some of these rate coefficients are shown in Table 11. It has however, been suggested that alternative physical mechanisms may be responsible for the decrease in hydrogen yield observed in the presence of scavengers93. 

TABLE 42. Calculated energy barriers (kcal mol 1) at QCISD//MP2 for the forward (A/i i ) and reverse (AIA ) hydrogen atom abstraction reactions of germanes by hydrogen atoms and various alkyl radicals R"... 

Since the Barton reaction and the Hofmann-Lofifler-Freytag reaction generate very reactive oxygen-centered and nitrogen-centered radicals respectively, the next 1,5- and 1,6-hydrogen atom abstraction reaction readily happens. However, 1,5-H shift does not proceed effectively by carbon-centered radicals, because there is not so much energy difference between the C-H bond before and after 1,5-H shift. So the reactions are quite limited. Eq. 6.21 shows iodine transfer from reactive 1-iodoheptyl phenyl sulfone (40) to a mixture of 5-iodoheptyl phenyl sulfone (41a) and 6-iodoheptyl phenyl sulfone (41b) initiated by benzoyl peroxide, through 1,5-H shift by an sp3 carbon-centered radical [56-58]. 

Phthalimides. Like their aromatic ketone counterparts, TV-alkyl phthalimides participate in hydrogen atom abstraction reactions to form a large variety of heterocyclic compounds. However, despite many similarities between phthalimides and aromatic ketones, there are some important differences. In contrast to aromatic ketones, electronically excited phthalimides are not transformed quantitatively into the triplet state, and thus they may react from both the singlet and the triplet state. In addition, phtalimides are much more inclined to photoinduced single-electron-transfer (PET) reactions [24,25]. (For details see Sec. 3.3.2.1.)... 

THE HYDROGEN ATOM ABSTRACTION REACTION FROM O-H BONDS... 

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