Radical species disproportionation

Carbonyl compounds can undergo various photochemical reactions among the most important are two types of reactions that are named after Norrish. The term Norrish type I fragmentation refers to a photochemical reaction of a carbonyl compound 1 where a bond between carbonyl group and an a-carbon is cleaved homolytically. The resulting radical species 2 and 3 can further react by decarbonylation, disproportionation or recombination, to yield a variety of products. 

Mino and Kaizerman [12] established that certain. ceric salts such as the nitrate and sulphate form very effective redox systems in the presence of organic reducing agents such as alcohols, thiols, glycols, aldehyde, and amines. Duke and coworkers [14,15] suggested the formation of an intermediate complex between the substrate and ceric ion, which subsequently is disproportionate to a free radical species. Evidence of complex formation between Ce(IV) and cellulose has been studied by several investigators [16-19]. Using alcohol the reaction can be written as follows ... 

Polymerisation does not continue until all of the monomer is used up because the free radicals involved are so reactive that they find a variety of ways of losing their radical activity. The two methods of termination in radical polymerisations are combination and disproportionation. The first of these occurs when two radical species react together to form a single bond and one reaction product as in Reaction 2.5. 

The last reaction commonly evoked to support the involvement of radical species 10 in tocopherol chemistry is the disproportionation of two molecules into the phenol a-tocopherol and the ort/zo-quinone methide 3 (Fig. 6.8), the latter immediately dimerizing into spiro dimer 9. This dimerization is actually a hetero-Diels-Alder process with inverse electron demand. It is largely favored, which is also reflected by the fact that spiro dimer 9 is an almost ubiquitous product and byproduct in vitamin E chemistry.28,29 The disproportionation mechanism was proposed to account for the fact that in reactions of tocopheroxyl radical 2 generated without chemical coreactants, that is, by irradiation, the spiro dimer 9 was the only major product found. 

The adsorbed cationic species are next transformed to the adsorbed radical species by the electron transfer. Finally, the Mg deposition may occur via the adsorbed radicals that disproportionate laterally on the electrode surface to form Mg metal and solution species, as represented, for example, by equations 35 ". 

Radicals being neutral species tend to react together. Indeed, the most common side reactions in free-radical processes involve the formation of adducts between two radicals, via combination or disproportionation. These unwanted termination steps usually occur much faster than the desired reactions between radicals and substrates. Thus, the key to control in both radical addition and polymerization procedures consists in lowering the concentration of transient radical species. This will minimize the side reactions between radical species, yet the kinetics of the useful reactions will also be affected. 

Propagation continues until the radical is terminated. Termination occurs when two radical species meet and react either by coupling or by disproportionation as shown in Fig. 15.12. 

As one can see from Figs. 1 and 2, the stability of ion-radical species is one of the most crucial requirements for the formation of conducting salts. Therefore, neutral donors and acceptors, as well as ion radicals, should not oxidize or reduce water, or react with oxygen. On the other hand, components of conducting materials should be chemically inactive, namely weak electrophiles or nucleophiles. Ion radicals must also exhibit sufficient degree of spin delocalization and be thermodynamically stable, to avoid, for example, dimerization or disproportionation. A sufficiently large energy gap should prevent the disproportionation of ion radicals into neutral and dica-tionic species. These requirements are presented schematically in Fig. 3. 

A systematic survey of the influence exerted by the substituents upon the stability of semiquinone forms was undertaken by Tozer and Dallas Tuck, in order to investigate the effect of the substituent upon the biological activity of phenothiazine derivatives, an activity correlated by many authors with in vivo formation of radical species. If the disproportionation reaction shown in Eq. (3) (Section IV, B, 1) were to reach equilibrium, the value K defined in Eq. (5) would express the stability of substituted S+ species. 

On treatment with water of radical species derived from unsubstituted and V-substituted phenothiazines, especially in alkaline medium, a mixture of 6-oxide and of the initial phenothiazine derivative is obtained, as a consequence of the disproportionation of S+ species into R and T" ", followed by hydrolysis of the cation thus formed. 

Radicals are species with at least one unpaired electron, which, in contrast to organic anions and cations, react easily with themselves in bond-forming reactions. In the liquid phase, most of these reactions occur with diffusion-controlled rates. Radical-radical reactions can be slowed only if radicals are stabilized by electronic effects (stable radicals) or shielded by steric effects (persistent radicals). However, these effects are not strong enough to prevent diffusion-controlled recombination of, for example, benzyl radicals or tert-butyl radicals.1 Only in extreme cases are the radical or di-tert-butylmethyl radical recombination rates low.2 While the recombination rates of the triphenyl-methyl radical is reduced due to both steric and radical stabilizing effects, the steric effect alone slows the recombination of the di-/t>/-/-butyl methyl radical. Since neither of the radicals have C-H bonds (I to the radical centre, disproportionation reactions, in which the hydrogen atom is transferred, cannot occur. 

Termination. The presence of small reactive radical species such as the methyl radical produced by fragmentation of the DCP initiator means that there may be a range of termination routes available. The bimolecular termination of graft radicals on different chains may result in a crosslink unless they disproportionate. The termination of the primary backbone radicals, R, will be competitive with monomer addition at low concentrations of M2 and again produce a crosslink. The possible competing crosslinking reactions are shown in Scheme 1.40. 

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