Mechanism of disproportionation

Russell proposed the mechanism of disproportionation of primary and secondary peroxyl radicals 2R1R2C1IOO —> R1 R2C(0) + o2 + r r2choh G. A. Russell [68]... 

The values of 2kt for secondary peroxyl radicals lies in the range 106 10s L mol-1 s 1 and that of tertiary peroxyl radicals are sufficiently less, i.e., in the range of 103—105 L mol-1 s 1. Cross-termination by the reaction of tert-R02 + vcr-R02 predominantly occurs according to the mechanism of disproportionation of secondary peroxyl radicals. 

Bunting and Kauffman (84CJC729) studied both the kinetics and mechanism of disproportionation and ferricyanide oxidation of 154 in aqueous base. Ferricyanide ion oxidation is kinetically first-order in each of ferricyanide ion... 

Biaglow JE, Kachur AV (1997) The generation of hydroxyl radicals in the reaction of molecular oxygen with polyphosphate complexes of ferrous ion. Radiat Res 148 181-187 Biaglow JE, Field KD, Manevich Y, Tuttle S, Kachur A, Uckun F (1996) Role of guanosine triphosphate in ferric ion-linked Fenton chemistry. Radiat Res 145 554-562 Bielski BHJ (1991) Studies of hypervalent iron. Free Radical Res Commun 12/13 469-477 Bielski BHJ, Allen AO, Schwarz HA (1981) Mechanism of disproportionation of ascorbate radicals. J Am Chem Soc 103 3516-3518... 

This interpretation of the mechanism of disproportionation is supported by two observations on crossed disproportionation reactions. In a kinetic study285 of the reduction of the 2-methylisoquinolinium cation by a series of2-benzyl-5-nitroisoquinolinium cations, the pH dependence of the reaction was observed to be consistent with hydride transfer from the pseudobase alkoxide ion 133 to the 2-methylisoquinolinium cation. Clark and Parvizi111... 

Combination and disproportionation are, of course, preceded by the occurrence of a different transition complex. It seems that the transition complex leading to the disproportionation of small radicals (ethyl) is more compact than that from which the combined complex is generated. Nevertheless, they both have a loosened structure. The mechanism of disproportionation is not known in detail. The high A factor indicates that a simple abstraction of the / hydrogen by one radical from the other is not probable. The transition complex may be polar [2] a direct proof is not accessible. The rate is determined by diffusion [3] the effect of solvent polarity on the activation parameters cannot be measured. The value of fct (comprising combination and disproportionation) of two small radicals is of the order of 109 mol-1 dm3 s l at temperatures around 273 K. 

In UTCHEM, the viscosity of the aqueous phase that contains the polymer is multiplied by the value of the polymer permeability reduction factor, F r, to account for the mobility reduction. In other words, water relative permeability, km, is reduced, whereas oil relative permeability, k , is sometimes considered almost unchanged. The reason is that polymer is not soluble in oU, so it will not reduce effective oil permeability. The mechanism of disproportionate permeability reduction is widely used in gel treatment for water shut-off. Many polymers and gels can reduce permeability to water more than to oil or gas. 

Equality of lengths of kinetic chains and the distance between interlocks of the networks forming in the reaction mixture can be considered as the kinetic condition of the onset of autoacceleration, or VR(qa) = ijc [63]. On the other hand, as the polymer solution concentration increases, the distance between interlocking contracts according to the relation i = (where i is the distance between interlocks in the polymer melt). Termination of a propagating chain at polymerisation of methacrylic monomers proceeds mostly by the mechanism of disproportionation that enables one, finally, to give the approximate equality Vr = P as ... 

The disproportionation reactions of metal nanoparticles could be observed in not only RTILs but also other electrolytes such as organic and aqueous solutions, so it is a significant issue to study the mechanisms in different media and obtain the possible interpretations. This chapter focuses upon understanding the effect and probable mechanism of disproportionation reactions in the formation of metal nanoparticles, especially Au, Pt, and Cu in RTILs. 

Gas-phase kinetics of NF2 disproportionation were measured at high initial NF2 pressures (up to 600 [2] or 1200 Torr [1]). A second-order rate law holds d[Np2]/dt=-k[NF2] with k = 10 exp(-98 kJ morVRT) cm -mor s" [1] (an expression k = 2.3x10 exp [-(23800 1200) cal morVRT] would better fit published values) or k= lO. - exp [-(36900 500) cal-morVRT] cm mor s" [2]. Largely different Arrhenius parameters are only the result of different experimental values at the low-temperature limit [2]. Additions of about 1200 Torr N2 or NF3 had no major effect on k. However, purification of N2F4 proved to be essential [2]. Preliminary rate data [5, 6] are probably influenced by NO impurities. The mechanism of disproportionation was believed to be ... 

A second Mobil process is the Mobil s Vapor Phase Isomerization Process (MVPI) (125,126). This process was introduced in 1973. Based on information in the patent Hterature (125), the catalyst used in this process is beHeved to be composed of NiHZSM-5 with an alumina binder. The primary mechanism of EB conversion is the disproportionation of two molecules of EB to one molecule of benzene and one molecule of diethylbenzene. EB conversion is about 25—40%, with xylene losses of 2.5—4%. PX is produced at concentration levels of 102—104% of equiHbrium. Temperatures are in the range of 315—370°C, pressure is generally 1480 kPa, the H2/hydrocatbon molar ratio is about 6 1, and WHSV is dependent on temperature, but is in the range of 2—50, although normally it is 5—10. 

The mechanisms of LiAlH4 reductions are more difficult to study. Eliel established that after the first reduction stage monoalkoxyaluminum hydride disproportionates to a tetraalkoxyaluminum hydride and LiAlH4. It... 

Which mechanism of termination will be preferably applied depends largely on the monomer used. Thus, methyl methacrylate chains terminate to a large extent by disproportionation, whereas styrene chains tend to termination by combination. The ratios of termination rate constants 8 = ktJkic (for disproportionation, td, combination,, c) are 5 == 0 and 5 = 2 for styrene [95] and methyl methacrylate [96], respectively. In the case of styrene, however, the values of 8 reported in the literature are at variance. Berger and Meyerhoff [97] found 8 = 0.2, at 52°C. Therefore, it is possible that a fraction of styrene terminates by disproportionation. 

Four studies suggest that k /kK has a significant temperature dependence (Table 5.5). Although not agreeing on the precise value of ktJkte, all four studies indicate that the proportion of disproportionation increases with increasing temperature. These results are at variance with model studies that suggest that kJkK is independent of temperature. It was also proposed that the preferred termination mechanism is solvent dependent and that disproportionation is favored in more polar media.161... 

Studies on VC polymerization are also complicated by the fact that only a small proportion of termination events may involve radical-radical reactions. Most termination is by transfer to monomer (Sections 4.3.1.2 and 6.2.63). Early studies on the termination mechanism which do not allow for this probably overestimate the importance of disproportionation.lb8 iw... 

Recombination of the ion radicals within the cage is thought of as forming the path to rearrangement whilst escape of the radicals and subsequent reaction with the hydrazo compound leads to the formation of disproportionation products often observed. The theory is mainly directed at the two-proton mechanism and does not accommodate well the one-proton mechanism, since this requires the formation of a cation and a neutral radical, viz. 

Gollnick and Stracke176 investigated the very complex mechanism involved in the photolysis of dimethyl sulphoxide and concluded that disproportionation is probably the route for the major sulphone-producing reaction. Other oxidized species such as methanesulphonic acid are also produced and are also probably formed by a series of disproportionation reactions, for example equation (62). Thus photolysis of dimethyl sulphoxide is not a synthetically useful reaction due to the large number of compounds produced. 

That this mechanism can take place under suitable conditions has been demonstrated by isotopic labeling and by other means. However, the formation of disproportionation and dimerization products does not always mean that the free-radical abstraction process takes place. In some cases these products arise in a different manner.We have seen that the product of the reaction between a carbene and a molecule may have excess energy (p. 247). Therefore it is possible for the substrate and the carbene to react by mechanism 1 (the direct-insertion process) and for the excess energy to cause the compound thus formed to cleave to free radicals. When this pathway is in operation, the free radicals are formed after the actual insertion reaction. 

A simple, high-yield procedure for the conversion of ArTlXj into ArjTlX compounds has recently been described 90). This symmetrization reaction, the mechanism of which is not known, can be effected by treatment of the ArTlX2 compound either with triethyl phosphite or with hot aqueous acetone. As a wide variety of ArTlXj compounds can now be easily prepared by electrophilic thallation of aromatic substrates with thallium(III) trifluoroacetate (q. v.), symmetrization represents the method of choice for the preparation of the majority of ArjTlX compounds. Only about twenty mixed compounds, RR TIX, have been prepared so far, and the only general synthetic procedure available consists of a disproportionation reaction between an RTIX2 species and another organometallic reagent [e.g., Eqs. (5)-(7)]. 

It should be emphasized that no spectroscopic evidence exists for either of the proposed species however, the lack of an EPR signal can be taken as strong evidence that no radicals are produced during activation of the catalyst with AlEts, thus favoring a kind of disproportionation mechanism. 

Electrochemical methods have been used extensively to elucidate the mechanism of reduction of tetrazolium salts. In aprotic media, the first step is a reversible one-electron reduction to the radical 154 as confirmed by ESR spectroscopy.256,266 As shown in Scheme 26, this radical can then disproportionate to the tetrazolium salt and the formazan anion (166) or take up another electron to the formazan dianion (167). The formation of the dianion through a direct reduction or through the intermediate tetrazolyl anion (168) has also been proposed.272-28 1,294 In aqueous solutions, where protonation/deprotonation equilibria contribute to the complexity of the reduction process, the reduction potentials are pH dependent and a one-electron wave is seldom observed. 

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