Disproportionation reversible

Aqueous polysulfide solutions are thermodynamically unstable with respect to thiosulfate and sulfide. Therefore, on heating to 150-240 °C under anaerobic conditions polysulfide ions disproportionate reversibly [72, 80, 104], e.g. ... 

The resulting trialkylboroxines disproportionate reversibly into trialkyl-borane and boric oxide upon heating above 200° C (166). The method was later improved based on the equilibrium of Eq. (82)... 

Spent Catalyst, Dissolution, Disproportionation, Reverse Precipitataion, Ferrosic Oxide. 

Germanium forms divalent compounds with all the halogens. Germaniunil 1) chloride can be prepared by passing the vapour of germanium(IV) chloride (see below) over heated germanium. The reaction is reversible and disproportionation of germanium(II) chloride is complete at about 720 K at atmospheric pressure ... 

The transalkylation reaction is essentiaHyisothermal and is reversible. A high ratio of benzene to polyethylbenzene favors the transalkylation reaction to the right and retards the disproportionation reaction to the left. Although alkylation and transalkylation can be carried out in the same reactor, as has been practiced in some processes, higher ethylbenzene yield and purity are achieved with a separate alkylator and transalkylator, operating under different conditions optimized for the respective reactions. 

The metallic monohaHdes zirconium chloride [14989-34-5] ZrCl, and zirconium bromide [31483-18-8] ZrBr, reversibly absorb hydrogen up to a limiting composition of ZrXH (131). These hydrides are less stable than the binary hydride ZrH2, and begin to disproportionate above 400°C to ZrH2 and ZrX in a hydrogen atmosphere (see also Hydrides). 

Disproportionation of Olefins. Disproportionation or the metathesis reaction offers an opportunity to convert surplus olefins to other desirable olefins. Phillips Petroleum and Institut Fransais du Petrc le have pioneered this technology for the dimerization of light olefins. The original metathesis reaction of Phillips Petroleum was intended to convert propylene to 2-butene and ethylene (58). The reverse reaction that converts 2-butene in the presence of excess ethylene to propylene has also been demonstrated (59). A commercial unit with a capacity of about 136,000 t/yr of propylene from ethylene via 2-butene has been in operation in the Gulf Coast since 1985 (60,61). In this process, ethylene is first dimerized to 2-butene foUowed by metathesis to yield propylene. Since this is a two-stage process, 2-butene can be produced from the first stage, if needed. In the dimerization step, about 95% purity of 2-butene is achieved at 90% ethylene conversion. 

The reverse reaction to give the gaseous species AlX(g) at high temperature accounts for the enhanced volatility of AIF3 when heated in the presence of A1 metal, and the ready volatilization of A1 metal in the presence of AICI3. Using calculations of the type outlined on p. 82 the standard heats of formation of the crystalline monohalides AIX and their heats of disproportionation have been estimated as ... 

Consequently, the coke cools down and the steam must be intermittently replaced by a flow of air to reheat the coke. At high temperatures, particularly in the presence of metal catalysts, CO undergoes reversible disproportionation ... 

The catalytic disproportionation of toluene (Figure 10-13) in the presence of hydrogen produces henzene and a xylene mixture. Disproportionation is an equilihrium reaction with a 58% conversion per pass theoretically possible. The reverse reaction is the transalkylation of xylenes with henzene ... 

Discussion. Iodine (or tri-iodide ion Ij" = I2 +1-) is readily generated with 100 per cent efficiency by the oxidation of iodide ion at a platinum anode, and can be used for the coulometric titration of antimony (III). The optimum pH is between 7.5 and 8.5, and a complexing agent (e.g. tartrate ion) must be present to prevent hydrolysis and precipitation of the antimony. In solutions more alkaline than pH of about 8.5, disproportionation of iodine to iodide and iodate(I) (hypoiodite) occurs. The reversible character of the iodine-iodide complex renders equivalence point detection easy by both potentiometric and amperometric techniques for macro titrations, the usual visual detection of the end point with starch is possible. 

The reaction between the PMMA and PS model radicals (4 and 5, generated from the unsymmetrical azo-compound 3) has been studied as a model for crosstermination in MMA-S copolymerization (Scheme 7.13).178,179 The value for tcross reaction was 0.56. In disproportionation, transfer of hydrogen from the PS model 5 to the PMMA radical 4 was ca 5.1 times more prevalent than transfer in the reverse direction (from 4 to 5). The value of kJklc(90°C) is between those of Atd/ tc(90oC) for the self-reaction of these radicals... 

Certain monomers may be able to act as reversible deactivators by a reversible addition-fragmentation mechanism. The monomers are 1,1-disubstituted and generate radicals that are unable or extremely slow to propagate or undergo combination or disproportionation. For these polymerizations the dormant species is a radical and the persistent species is the 1,1 -disubstituted monomer. 

The relative importance of the disproportionation process (SET between two anion radicals) depends principally on the thermodynamic constant (K). It can be easily determined more or less accurately from the potential difference existing between the first cathodic peak and the second one. (An exact calculation would be possible from the thermodynamic potentials of the two reversible transfers in the absence of proton sources and at reasonable sweep rates so as to inhibit any undesirable chemical reaction.)... 

Aqueous plutonium photochemistry is briefly reviewed. Photochemical reactions of plutonium in several acid media have been indicated, and detailed information for such reactions has been reported for perchlorate systems. Photochemical reductions of Pu(VI) to Pu(V) and Pu(IV) to Pu(III) are discussed and are compared to the U(VI)/(V) and Ce(IV)/(III) systems respectively. The reversible photoshift in the Pu(IV) disproportionation reaction is highlighted, and the unique features of this reaction are stressed. The results for photoenhancement of Pu(IV) polymer degradation are presented and an explanation of the post-irradiation effect is offered. 

The effects of UV light on the disproportionation of Pu(IV) The increased rate is demonstrated by the rapid decrease in Pu(IV), and corresponding increase in Pu(III) and Pu(IV). A reversible shift in the equilibrium is obvious from the return to dark condition concentrations after each exposure. The effects of increasing temperature are compared (3). 

Later experiments (4 ) were designed to determine a cell e.m.f. for the plutonium disproportionation system with a particular light source. Concentration quotients for the light and dark conditions, Qg and Qj, were determined, and an energy difference of 1.65 kcal (32 mV) was calculated by the relation -RTln C /Qd This reversible photochemical shift may be the only single-element system known at this time and certainly is the simplest such system. Even though the radioactive properties could prevent development and utilization of a plutonium photoconversion system, these studies certainly suggest that similar nonradioactive and more acceptable systems could be discovered and developed. 

The dimeric metallo-ester slowly disproportionates in the absence of monomer forming methyl methacrylate (the monomer, M) and the monomeric metallo-ester, the reverse reaction by which the dimer could be formed from the monomeric metallo-... 

The reverse reaction of the disproportionation equilibrium has been investigated by Masters and Schwartz. It provides a pathway for the exchange of U(VI) and U(IV) as has been previously mentioned. 

However, this reaction is not possible thermodynamically , because < 0.6 and 1/4 1.4 V on the contrary, the reverse reaction, i.e. the disproportionation of chromium(V), is favoured. This and other objections make it necessary to modify the reaction scheme for this system. It is recommended that steps (18)-(24) be replaced by... 

When arenesulfonyl radicals are generated in benzene the only reported products are those of disproportionation (vide supra). However, Camaggj and eoworkers have found that arenesulfonyl radicals in halobenzene replace the halogen atom at 150-190 °C, the relative reactivities being for Cl Br I, 1 5.9 18.6. These authors proposed that the reaction proceeds via a reversible ipso-substitution. 

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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