Radical anion formation relative reactivities

The factors affecting the relative reactivity of aryl halides in SrnI reactions have been analysed and compared645. Competition experiments of pairs of substrates, in photo-stimulated reactions with pinacolone enolate ion in liquid ammonia, reveal a spread of reactivity exceeding three powers of ten. The ease of formation of the radical anion of the substrate appears to dominate the overall reactivity. The rate of dehalogenation of the radical anion may become important when its stability exceeds a certain threshold. When the fragmentation rate of the radical anion intermediate is fairly slow, the overall reactivity diminishes. 

In the photostimulated reaction of 2-naphthoxide ion (198) with an o-dihalobenzene, an aromatic a radical may be formed very close to the oxygen functionality along the chain propagation cycle of the S l mechanism. This spatial proximity and the fact that the intramolecular coupling between the two moieties will form a relatively stable radical anion will favour the reaction between both reactive centres. Thus in the photostimulated reaction of o-dihalobenzenes with 198 in liquid ammonia, the formation of the monosubstitution 351 and of the cyclization product 352 were reported in yields that depend on the substrate and on the reaction conditions (equation 205)346. 

It is remarkable that the formation of phenoxyl radicals was observed only in the pH range close to the p a of HO radicals (p a H -9) and of H2O2 (p a H -7). As shown in Fig. 4, this behavior reflects the relative rate of formation of the superoxide radical anion, 02 , which is enhanced by the facile hydrogen abstraction from H02 by H0 , but suppressed at higher pH when HO is deprotonated. Furthermore, singlet oxygen is formed from 02 by reaction with HO or with 0 the latter is another very reactive species that can lead to severe membrane damage via various routes, provided these extreme pH conditions are reached. 

Analogously, although the enhanced reactivity of quinoline derivatives (relative to pyridine counterparts) in nucleophilic additions such as the formation of pseudobases and Reissert compounds probably relates to the relatively lower energy of the LUMO in the quinoline derivatives, the radicals from these heterocycles, in which the corresponding orbitals are singly occupied, are anionic thus, reactivity toward nucleophiles is also unlikely to be important. 

The most important conclusion from stopped-flow studies is that the rate constants of propagation of several styrene derivatives are approximately kp 105-1 mol, L sec I at 0° C, which is relatively high compared with those of radical and anionic systems (average kp == 102 mol 1L-sec 1 at 0° C). Solvent effects are noticeable, with propagation slower in more nucleophilic 1,2-dichloroethane [17] than in CH2Cl2 [18] under comparable conditions. That is, the carbenium ion reactivity is apparently reduced by interaction with more nucleophilic solvents. However, such interactions do not result in formation of chloronium ions, whose spectra would be very different compared to those of the corresponding carbenium ions. 

Superoxide anion is a relatively nonreactive species in aqueous solution, but in the presence of hydrogen peroxide and a transition metal such as iron, the extremely reactive hydroxyl radical may be generated through a superoxide anion-driven, metal-catalysed Fenton reaction. The hydroxyl radical is able to initiate lipid peroxidation directly through ab.straction of hydrogen from fatty acids leading to the formation of off-flavors and other undesirable compounds. 

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