Hydrogen atom transfer rate

A correlation has been found between 0-H bond strength and the hydrogen atom transfer rate (Figure V111.2). 

Our group was able to obtain hydrogen atom transfer rate constants (Table 1) with typical organic solvents and their Arrhenius Activation parameters (Table... 

Predicting Organic Hydrogen Atom Transfer Rate Constants... 

A photoinduced hydrogen atom transfer in cw-l-(2-pyrrolyl)-2-(2-quino-line)ethene was reported (94JA3171).The rate eonstant A (5 —> 4) increases with increasing temperature from 2.1 10 s at 15.8°C to 7.7 10 s at 39.5°C, giving an aetivation energy of 9.4 keal/mol. 

The kinematic theory can now be used to determine the appropriate values of Js, lQtotal, and It (Equations 15, 16, and 17, respectively) for various assumed values of k, the rate constant of Reaction O—a quantity about which nothing is known—and for the various mechanisms. Obviously from the chemistry of the system, Reactions U, V, and W cannot occur by a hydrogen atom transfer mechanism hence, only two cases need be considered—Reactions S, T, U, V, and W, occurring by... 

In deuterated solvent the rate of exchange was found to be lower than in aqueous media the ratio k H20) k D20) was found to be dependent on the acid concentration. Sullivan et al. have suggested a hydrogen atom transfer process and a water bridging process... 

PEDRIELLI p, HOLKERi L M and SKIBSTED L H (2001b) Antioxidant activity of (+)-catechin. Rate constant for hydrogen atom transfer to peroxyl radicals, Eur Food Res Technol, 213, 405-8. 

Methylmalonyl-CoA mutase (MCM) catalyzes a radical-based transformation of methylmalonyl-CoA (MCA) to succinyl-CoA. The cofactor adenosylcobalamin (AdoCbl) serves as a radical reservoir that generates the S -deoxyadenosine radical (dAdo ) via homolysis of the Co—C5 bond [67], The mechanisms by which the enzyme stabilizes the homolysis products and achieve an observed 1012-fold rate acceleration are yet not fully understood. Co—C bond homolysis is directly kineti-cally coupled to the proceeding hydrogen atom transfer step and the products of the bond homolysis step have therefore not been experimentally characterized. 

Enthalpies, Activation Energies, and Rate Constants of Intramolecular Hydrogen Atom Transfer in Peroxyl Radicals Calculated by the IPM Method [36], (Ais Increment of Polar Interaction in the Transition State)... 

The mechanistic proposal of rate-limiting hydrogen atom transfer and radical recombination is based on the observed rate law, the lack of influence of CO pressure, kinetic isotope effects [studied with DMn(CO)s] and CIDNP evidence. In all known cases, exclusive formation of the overall 1,4-addition product has been observed for reaction of butadiene, isoprene and 2,3-dimethyl-l,3-butadiene. The preferred trapping of allyl radicals at the less substituted side by other radicals has actually been so convincing that its observation has been taken as a mechanistic probe78. 

Sulfenic acids undergo hydrogen atom transfer to free radicals extremely readily (Koelewijn and Berger, 1972), rate constants of at least 107 M-1 s-1 being observed for (18a) when R is a peroxy radical. Block and O Connor (1974a) believe the marked antioxidant activity of most thiolsulfinates is... 

Aryl substitution on germanium, whether single or multiple, has only a small effect on the rate constants for hydrogen atom transfer, whereas the rate constant increases substantially with substitution of an alkyl group on Ge by a silyl group, much as observed with the silanes. A strong substituent effect also was observed for germane 19. 

DHS can photosensitize reactions involving hydrogen atom transfer, which likely involve triplet state intermediates. For example, hydrogen transfer from the nitrogen of aniline to the sensitizer occurs at much higher rates than observed in the aniline photoreaction in distilled water. 

Although autoxidation of Ru(sar) + has similar characteristics in acidic solution, in base hydrogen atom transfer from Ru(sar) + to O2 leads to a deprotonated Ru(III) species which is oxidized to relatively stable Ru" (sar-2 H+) + Ref. 175. The strong deviation from linearity for semi-log plots, with a large excess of O2, is removed when Fe(II) is added. This suppresses the step and doubles the rate. Compare Sec. 2.2.1(b). The value of k can be assessed as 1.3 x 10 M s Ref. 176. The behavior of pentacyanoruthenium complexes has been compared with the iron analogs. Substitution in M"(CN)5L" with both M = Fe and Ru is dissociative, with decreased lability for tbe Ru(II) species. Table 8.10. 

Hydrogen Atom Transfer from Hydroperoxides to Peroxy Radicals. The reaction of cumylperoxy radicals with Tetralin hydroperoxide (Reaction 10) can be studied at hydroperoxide concentrations below those required to reduce the oxidation rate to its limiting value. The rate of oxidation of cumene alone can be represented by ... 

This process has not been studied in detail. It has been shown that diphenylnitren-ium ion reacts with various hydrocarbons and metal hydrides to give diphenyl amine. An analysis of the rate constants for these processes showed that the reaction was most likely a hydride transfer, rather than a hydrogen atom transfer (Fig. 13.56). Novak and Kazerani found a similar process in their study of the decay reaction of heteroarylnitrenium ions. 

The HCo(CO)4 complex is therefore inferred to be involved in initial hydrogen transfer to carbon monoxide. This step was initially proposed to comprise rate-determining hydrogen atom transfer from HCo(CO)4 to free CO, affording a formyl radical, HtO subsequent reaction with further HCo(CO)4 would lead to the observed products (35). However, kinetic observations (the zero-order dependence on CO partial pressure) were later made which are inconsistent with such a process (36). 

Participation of the hydride-formyl equilibrium in (16) is also plausible in light of an apparent inverse kinetic deuterium isotope effect for the catalytic process. Use of deuterium gas instead of hydrogen (cf. Expts. 6 and 4 in Table II) causes an increased rate, with kH/k = 0.73 (37). The existence of an isotope effect implies that hydrogen atom transfer occurs before or during the rate-determining step, and an inverse kinetic isotope effect may be possible in the case of a highly endothermic, product-like transition state (73). On the other hand, Bell has concluded that inverse kinetic isotope... 

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