Mechanism, radical hydrogen

Chemical combustion is initiated by the oxidation or thermal decomposition of a fuel molecule, thereby producing reactive radical species by a chain-initiating mechanism. Radical initiation for a particular fuel/oxygen mixture can result from high-energy collisions with other molecules (M) in the system or from hydrogen-atom abstraction by 02or other radicals, as expressed in reactions 6.1-6.3 ... 

Radicals for addition reactions can be generated by halogen atom abstraction by stannyl radicals. The chain mechanism for alkylation of alkyl halides by reaction with a substituted alkene is outlined below. There are three reactions in the propagation cycle of this chain mechanism addition, hydrogen atom abstraction, and halogen atom transfer. 

Nitrosoarenes are readily formed by the oxidation of primary N-hydroxy arylamines and several mechanisms appear to be involved. These include 1) the metal-catalyzed oxidation/reduction to nitrosoarenes, azoxyarenes and arylamines (144) 2) the 02-dependent, metal-catalyzed oxidation to nitrosoarenes (145) 3) the 02-dependent, hemoglobin-mediated co-oxidation to nitrosoarenes and methe-moglobin (146) and 4) the 0 2-dependent conversion of N-hydroxy arylamines to nitrosoarenes, nitrosophenols and nitroarenes (147,148). Each of these processes can involve intermediate nitroxide radicals, superoxide anion radicals, hydrogen peroxide and hydroxyl radicals, all of which have been observed in model systems (149,151). Although these radicals are electrophilic and have been suggested to result in DNA damage (151,152), a causal relationship has not yet been established. Nitrosoarenes, on the other hand, are readily formed in in vitro metabolic incubations (2,153) and have been shown to react covalently with lipids (154), proteins (28,155) and GSH (17,156-159). Nitrosoarenes are also readily reduced to N-hydroxy arylamines by ascorbic acid (17,160) and by reduced pyridine nucleotides (9,161). 

Colquhoun and Schumacher [98] have shown that y-linolcnic acid and eicosapentaenoic acid, which inhibit Walker tumor growth in vivo, decreased proliferation and apoptotic index in these cells. Development of apoptosis was characterized by the enhancement of the formation of reactive oxygen species and products of lipid peroxidation and was accompanied by a decrease in the activities of mitochondrial complexes I, III, and IV, and the release of cytochrome c and caspase 3-like activation of DNA fragmentation. Earlier, a similar apoptotic mechanism of antitumor activity has been shown for the flavonoid quercetin [99], Kamp et al. [100] suggested that the asbestos-induced apoptosis in alveolar epithelial cells was mediated by iron-derived oxygen species, although authors did not hypothesize about the nature of these species (hydroxyl radicals, hydrogen peroxide, or iron complexes ). 

This approach was the first application of non-enediyne carbon centered radical mediated DNA cleavage agents that were not only capable of binding to DNA but could also be sequence specific. Further work is still needed to elucidate and confirm the sites of cleavage, nature of binding of these molecules and the mechanism of hydrogen abstraction from the nucleic acid backbone. 

In the HAT mechanism, a hydrogen atom is transferred from the -carbon of the amine to FAD to form a carbinyl radical and FADH semiquinone radical. FADH semiquinone radical abstracts the unpaired electron from the carbinyl radical forming the imine and FADH2 after protonation. 

The associative mechanism resembles a conventional radical (hydrogen atom) substitution reaction where the 7T-bonded benzene molecule is attacked by a hydrogen atom formed by the dissociative adsorption of water or hydrogen gas. The activation energy in this process is essentially due to the partial localization of one tt electron in the transition complex 21, 31). The transition state differs, however, from conventional substitution reactions by being 77-bonded to the catalyst surface ... 

Cyclohexyl xanthate has been used as a model compound for mechanistic studies [43]. From laser flash photolysis experiments the absolute rate constant of the reaction with (TMS)3Si has been measured (see Table 4.3). From a competition experiment between cyclohexyl xanthate and -octyl bromide, xanthate was ca 2 times more reactive than the primary alkyl bromide instead of ca 50 as expected from the rate constants reported in Tables 4.1 and 4.3. This result suggests that the addition of silyl radical to thiocarbonyl moiety is reversible. The mechanism of xanthate reduction is depicted in Scheme 4.3 (TMS)3Si radicals, initially generated by small amounts of AIBN, attack the thiocarbonyl moiety to form in a reversible manner a radical intermediate that undergoes (3-scission to form alkyl radicals. Hydrogen abstraction from the silane gives the alkane and (TMS)3Si radical, thus completing the cycle of this chain reaction. 

Peroxidases (EC 1.11.1.7), which have ferric protoheme prosthetic groups, react non-selectively via free radical mechanisms, using hydrogen peroxide as the electron acceptor. A reactive Fe(IV)-0 species and a radical heme intermediate are formed, and the intermediate then reacts with the reducing substrate to produce the oxidized product, regenerating the Fe(III) ion. 

The photoreactions of a-dicarbonyl compounds are quite different in the vapor and condensed phases. In the vapor phase, carbon-carbon bond cleavage is the preferred mode of reaction but in the condensed phase, many of the observed reactions can be rationalized by a mechanism involving hydrogen abstraction. Internal hydrogen abstraction, when possible, is generally preferred over abstraction from the solvent. With the exception of diethyl oxalate, which undergoes photoreactions typical of an ester, only those compounds that are reasonably strained or can yield reasonably stable free radicals give decarbonylation products. In the presence of suitable substrates, cycloaddition reactions have also been observed. 

The self-peroxidation reactions of Cu,Zn-SOD provide a particularly novel mechanism for the formation of protein radicals. Hydrogen peroxide is generated during the reaction cycle of the enzyme ... 

The intermediate remains on the electrode until it is transformed into another particle during the consecutive steps that make up the overall reaction. The simplest example is (Section 7.6.2) the mechanism of hydrogen evolution, in which one possible step involves chemical recombination between adsorbed H s, put onto the electrode surface by means of the discharge of H20+ from acid or H20 from alkaline solutions. The adsorbed H is the intermediate radical. 

It has been known that the electrolysis in an MeCN- NaClO system generates an acid The hydrogen has to originate from the solvent. A mechanism for hydrogen abstraction from acetonitrile by the electrooxidatively generated radical 104- to produce perchloric acid has been proposed, but no evidence for the succinonitrile formation appeared (Eq. (5)). The detection of the 104- radical by the aid of HSR was tried But it was found to be difficult to differentiate between the perchlorate radical and the radical from chlorine dioxide The electrolysis in a CH Clj—... 

The last three entries in Table 13 reflect a marked preference for a-methylene versus methyl proton transfer for electron-withdrawing a substituents. These amines are also unusual in that they react with t in nonpolar solvents and do not display exciplex fluorescence. While this unusual behavior was initially attributed to a free radical hydrogen atom transfer mechanism leading to the formation of exceptionally stable "merostabilized" a-aminoallyl radicals (115), our current view is that the high kinetic acidity of the a-C-H bond of these amines when complexed with t is responsible for their behavior. 

The transfer of hydrogen to the alkene also occurs by a radical process,55,50 and not by the insertion reaction which is much more common in homogeneous hydrogenation. These kinetic studies were carried out using cinnamic acid as alkene. The observation of alkenyl and allyl species in diene hydrogenation suggests that a different mechanism of hydrogen transfer may... 

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