Mechanism disproportionation

Dehydrochlorination of 1,1,2-trichloroethane at 500°C in the presence of a copper catalyst gives a different product, ie, cis- and /n7 j -l,2-dichloroethylene. Addition of small amounts of a chlorinating agent, such as chlorine, promotes radical dehydrochlorination in the gas phase through a disproportionation mechanism that results in loss of hydrogen chloride and formation of a double bond. The dehydrochlorination of 1,2-dichloroethane in the presence of chlorine, as shown in equations 19 and 20, is a typical example. 

As in the case of PS (Section 8.2.1) polymers formed by living radical polymerization (NMP, ATRP, RAFT) have thermally unstable labile chain ends. Although PMMA can be prepared by NMP, it is made difficult by the incidence of cross disproportionation.42 Thermal elimination, possibly by a homolysis-cross disproportionation mechanism, provides a route to narrow polydispersity macromonomers.43 Chemistries for end group replacement have been devised in the case of polymers formed by NMP (Section 9.3.6), ATRP (Section 9.4) and RAFT (Section 9.5.3). 

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. 

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. 

Binuclear halogenide clusters with ammonium cations under quasi-isother-mal conditions also decompose by the disproportionation mechanism (33) [51,123]. 

The thermal decomposition of binuclear technetium sulfate clusters also occurs according to the disproportionation mechanism, but in this case, (a) other technetium-containing products are formed, and (b) a weight loss due to the evolution of gaseous products is also observed (36) [59]. 

Evidence for a competing disproportionation mechanism (see Figure 1) for the termination of chain ends is provided by the combined presence of the peaks from 4 and 5 in the MALDI-TOF mass spectrum of this PMMA polymer (see Figure 6) [10]. Confirmation of the presence of the unsaturated and saturated chain ends, arising from disproportionation, was obtained by means of and 13C NMR spectroscopy, respectively [11]. 

The fragmentation in the MALDI-TOF MS experiment noted from the brominated polymer is less prevalent in this chlorinated material, but evidence for the lactone-ended oligomer (7) is still noted (see Figure 8). Evidence for the competing disproportionation mechanism is again provided by the presence of peaks proposed to be from unsaturated (8) and saturated (9) end-functionalised oligomer [10]. The data indicate that this disproportionation reaction is a minor termination pathway, as low intensity peaks are noted for 8 and 9 (as for 4 5 from the brominated analogue). 

Figure 4. Acid catalyzed xylene disproportionation mechanism.

Ion-pairing effects may considerably influence disproportionation mechanisms that involve homogeneous redox reactions of anions to their respective dianionic and neutral species (Eq. 4) [52, 53],... 

Disproportionation mechanisms have been proposed for protonahon reachons and intramolecular rearrangements (see Sects. 4.3.2 and 4.3.3) [54]. The prominent feature is that follow-up processes at the level of the dianion can already take place at potentials corresponding to radical anion formation. In order to evaluate data for disproportionation reactions it is necessary to know the value of the disproporhonahon equilibrium constant. 

Such a process would have an intensity exponent of unity as observed. A further fact in support of the disproportionation mechanism is that the yield of CF2CI2 is largely independent of the ketone pressure at room temperature an abstraction mechanism would require a first-order dependence upon ketone pressure. While the dimer of CF2, tetrafluoro-ethylene, has never been observed in the reaction mixture, a white solid collected in the cell which was probably a polymer of CF2. While the experimental conditions are not strictly comparable, it is significant that the absorption spectrum of CF2 has been observed in the flash photolysis of 1,3-dichlorotetrafluoroacetone.39 When the temperature is raised, however, the yield of CF2C12 in normal photolysis, increases rapidly suggesting an energy of activation and this process can only be chlorine abstraction. The rate function ... 

The dichlorodifluoromethane and trichlorofluoromethane identified in the photolysis products could be formed either by an abstraction or a disproportionation mechanism as discussed previously. The intensity exponents are both close to unity at room temperature, suggesting that the disproportionation mechanism is predominant but it is not possible to say whether CFC1 is formed or only CF2. 

It is evident that a single electron transfer photoproduct is transformed into a doubly reduced charge relay in two phase systems. The primary processes in the natural photosynthetic apparatus involve single electron transfer reactions that proceed in hydrophobic-hydrophilic cellular microenvironment. Thus, we suggest similar induced disproportionation mechanisms as possible routes for the formation of multi-electron charge relays, effective in the fixation of CO2 or N2. 

The mechanism of the unprecedented chromium-catalysed selective tetramerization of ethylene to oct-1-ene has been investigated. The unusually high oct-1-ene selectivity of this reaction apparently results from the unique extended metallacyclic mechanism in operation. Both oct-1-ene and higher alk-l-enes were formed by further ethylene insertion into a metallacycloheptane intermediate, whereas hex-1-ene was formed by elimination from this species as in other trimerization reactions. Further mechanistic support was obtained by deuterium labelling studies, analysis of the molar distribution of alk-l-ene products, and identification of secondary co-oligomerization reaction products. A bimetallic disproportionation mechanism was proposed to account for the available data.120... 

However, the thermal decomposition of Ph4SbOSiPh3 has been explained by a disproportionation mechanism and following products have been isolated... 

During the oxidative formation of radicals of type 127 from the appropriate tetrahydrolumazine, various reactivities were found.399 Sequential oxidation in formic acid yielded 127 followed by 128, even when insufficient oxidant was used, implying a disproportionation mechanism. Oxidation of JV(J)-alkyl side chains to give N(J)-acyl substituents was also suggested to account for a further oxidation mode observed.399 Pfleiderer and co-workers have elucidated radical-mediated pathways in polarographic studies of pterins.400... 

Another general problem, perhaps with somewhat more impact on homogeneous solution chemistry, is whether the reactions of radical ions with nucleophiles or electrophiles, depending upon the charge, go directly via the radical ions or if reactive doubly charged ions must first be formed by disproportionation. The reaction of thianthrene radical cation with water (Murata and Shine, 1969 Shine and Murata, 1969) is a typical example which was proposed to involve the disproportionation mechanism (71 and 72). The... 

The complex mechanism of the protonation of anion radicals in DMF may have some bearing upon the mechanism proposed for the reaction of perylene radical anion with alcohols (Levin et a/., 1972) and anthracene radical anion with t-butyl alcohol (Rainus et a ., 1973). The disproportionation mechanism was proposed for these reactions in ethereal solvents with alkali-metal counter-ions. The principal evidence for the mechanism was the observation of rate laws of the form of (77) where was suggested to be... 

A closer examination was made of the mechanism of deprotonation of HMB radical cation in acetonitrile. Tables 7 and 8 show the effect of substrate concentration and temperature, respectively, on the apparent rate constants for the deprotonation of the radical cations of HM B and HMB-dig measured by DCV (Parker, 1981b). Although data for both substrates gave a very good fit to theoretical data for the disproportionation mechanism, the observed rate constants were concentration dependent. This indicates that Raib is greater than 1 and less than 2 suggesting a complex mechanism. The com-... 

SCHEME 4. Disproportionation mechanism of L Cu(fi-diketonate) for Cu film deposition on substrates. Reproduced from Reference 1 by permission of Wiley-VCH... 

As the functionality of the (acrylic ester-diene) copolymers l8,19> is higher than two, the disproportionation mechanism is unimportant and may be neglected. That means that in the diazo-initiated polymerization, termination mostly takes place by recombination. 

Klug-Roth D, Fridovich I, Rabani J. (1973) Pulse radiolytic investigations of superoxide catalyzed disproportionation. Mechanism for bovine superoxide dismutase. J Am Chem Soc 95 2786-2790. 

It has been recently demonstrated [4] that the disproportionation mechanism of n-propylbenzene essentidly depends on the nature of the catalyst it is mainly of Sn2 type on Y-zeolites and leads to a meta/para (m/p) ratio s 2. This has been also observed for sBB under atmospheric pressure [5]. The ratio m/p between the two di-sec-butylbenzene isomers can be assumed as an indication of the relative extent of the Sn2 and SnI mechanisms the higher the m/p ratio, the higher the Sn2 extent. At low time factor values, when DBA is present in small amount (Fig.2a), the m/p ratio observed is about 1.6 and is constant with time on stream (t-o-... 

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