Hydrogen atom reactivity relationships

At this point let us briefly consider the relationship between the carbonyl triplet state and another system capable of hydrogen atom abstraction alkoxy radicals. A comparison of the differences and/or similarities between the reactivity of the carbonyl triplet and that of an alkoxy radical should indicate whether the triplet state behaves as a normal ground state radical or if electronic excitation imparts unique properties leading to reactions not characteristic of ground state radicals. 

Paquette has reported the most detailed investigation of this sort, of the relationship between (crystal) structure and reactivity for a series of compounds [78] (Paquette et al., 1990). These undergo a thermal pericyclic reaction in which two hydrogen atoms are transferred between alkene and alkane C-C bonds across a gap whose dimensions can be accurately measured, and can be varied by varying the group X bridging the central... 

Hydroxyl radicals, of course, react with organic compounds either by abstraction (of a hydrogen atom or electron) or addition pathways. Both of these routes result in the formation of new free-radical species that are usually quite reactive toward molecular oxygen, invariably present in AGP systems. The intermediate peroxy radicals participate together with the other reactive intermediates in the system to establish a mechanistic relationship (Staehelin and Hoigne, 1985 Peyton et al.,... 

Experimental data indicate that the anti pathway for E2 reactions is favored over the syn pathway. In one of the earliest studies of the stereochemistry of the E2 reaction, Cristol found the rate constant for the dehydrochlorination of the )3 isomer of benzene hexachloride (1,2,3,4,5,6-hexachlorocyclohexane, 6), in which each chlorine is cis to the hydrogen atoms on either side of it, to be only 10" times those of the other benzene hexachloride isomers. Since each of the other isomers has at least one hydrogen atom trans to a chlorine atom on an adjacent carbon atom, the low reactivity of 6 suggested that the E2 reaction occurs preferentially when there is a trans relationship for the hydrogen atom and chlorine atom on cyclohexane. ... 

Conventional free radical chain oxidation (referred to as autoxidation) involves chain initiation, propagation, and termination steps [15-17]. Thepossibility of hydrogen atom abstraction by radical species in arenes without alkyl side chains is small because of the strong sp C—H bonds (the dissociation energies of the sp Ar—H bonds and sp ArCR —H are 112 and ca. 90kcal/mol, respectively [18]). Phenols readily react by direct abstraction of H from the hydroxyl gronp to form phenoxyl radical ArO, and this pathway is implicated in their antioxidant properties [16, 19]. Numerous literature addresses structure-reactivity relationships in the chemistry of phenols and phenoxyl radicals (see Refs. 1, 16, 19 and references therein). 

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