Atom abstraction process

The oxidation of di-2-chloroethyl ether is first-order with respect to ether, but is autocatalytic and chloride ion is liberated. A hydrogen atom abstraction process, similar to that above, probably takes place, viz. 

The LFP techniques make it possible to observe directly the hydrogen atom abstraction process by triplet carbenes from appropriate H donors. The techniques also give useful kinetic information. 

Finally, with the analogous substituted butyrophenones, Pitts has reported that no such solvent effect on reactivity is observed in the intramolecular H-atom abstraction process although the absorption spectra of these ketones show pronounced solvent shifts similar to the corresponding benzophenones. 

An understanding of the factors that influence the rates of hydrogen atom abstraction processes is very important in order to maximize the utility of radical-based processes in carbon-carbon bond-forming reactions. This is because most such reactions are chain reactions in which one of the key propagation steps involves transfer of a hydrogen atom from some hydrogen atom transfer agent, such as tri-n-butyltin hydride. 

There are two basic methods for making polymer materials photo-chemically degradable.1,2 One method is to chemically incorporate a chromo-phore into the polymer chains. One commercially successful chromophore is the carbonyl group.1,2,7 Absorption of UV radiation leads to degradation by the Norrish type I and II processes or by an atom abstraction process (Scheme 1). Note that once radicals are introduced into the system, chain degradation occurs by the autoxidation mechanism (Scheme 2). 

This reaction leads to ketyl radicals formed by the hydrogen atom abstraction process. The presence of ketyl radicals of the type (CH3)aCOH in the reaction mixture can be demonstrated by the formation of... 

The light-induced addition of 2-pyrrolidone to olefins when induced directly by light leads to traces only of the desired 1 1 adducts. On the other hand, when acetone is used as a photosensitizer for this reaction it results in relatively high yields of the 1 1 adducts and may be exploited for synthetic purposes in some cases. As in the case of y-butyrolactone the initiation step can be regarded involving a hydrogen atom abstraction process from the lactam by the excited acetone molecule,... 

Their reaction with H, on the other hand, is an atom abstraction process (12) rather than electron transfer. In the case of chloro-... 

The synthetic potential of fhe intermolecular radical additions would be dramatically enhanced by developing conditions compatible wifh primary radicals. Less stable T radicals (versus 2° or 3°) often suffer premature reduction by hydrogen atom abstraction processes and are impractical under Ets B/O2 initiation conditions due to fhe competition with ethyl radicals. These considerations led us to seek alternatives to triethylborane. 

At 298 K, kinetic studies indicate that the H-atom abstraction processes account for 9% of the overall OH radical reaction for phenol [via pathway (65b)] and 7% of the overall OH radical reaction for o-cresol [via pathways (65a) and (65b)] (Atkinson, 1989). 

Detailed investigations of all these reactions show they proceed by an electron-transfer hydrogen-atom-abstraction process ... 

Rate constants for the above H-atom abstraction process are on the order of 10 s with the actual values depending both on the nature of the thiol... 

Singh et al. (2003) put forward a more latest predictive model (design) for the cytochrome P-450 (CYP) 3A4 metabolism. This method exclusively rests upon the primary lateral sclerosis (PLS), however, one of the descriptors is totally based on acute myocardial infarction (AMl)-calculated H-atom abstraction process. 

Rate constants for the above H atom abstraction process are on the order of 10 M" s with the actual values depending both on the nature of the thiyl and the PUFA. For a given thiyl radical the measured rate increases with the number of bisallylic hydrogens. For the reaction of the thiyl radical from glutathione, GS, with PUFAs, for example, the rate constants increase from 0.8 x lO M" s (linoleic acid, 18 2, two double bonds in one bisallylic functionality), over 1.9 x 10 M" s (linolenic acid, 18 3, three double bonds in two bisallylic functionalities), to 3.1 X 10 M s (arachidonic acid, 20 4, four double bonds in three bisallylic functionalities). As expected, the mere length of the aliphatic chain does not play any role, as indicated by identical rate constants for the 18 3 and 22 3 PUFAs. Also, practically no reaction k < 10 M" s" ) is observed for the corresponding reaction of RS with oleic acid (18 1, one double bond) which lacks bisallylic hydrogens. 

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