Disproportionation thermodynamics

Complexes of Cu halides with various oxazole or thiazole ligands can be used to produce copper images from physical developer baths in two ways. The first is conventional reduction by a reducing agent such as hydroquinone or ascorbic acid, generally in the presence of an amine base such as dimethylaminoethanol. Ag or Pd nuclei catalyze the process. In the second method, Ag or Pd catalyzes the disproportionation of the Cu complexes into Cu and Cu . A complexing agent for Cu such as triethylenetetramine is added to make the disproportionation thermodynamically favorable. ... 

Selective Toluene Disproportionation. Toluene disproportionates over ZSM-5 to benzene and a mixture of xylenes. Unlike this reaction over amorphous sihca—alumina catalyst, ZSM-5 produces a xylene mixture with increased -isomer content compared with the thermodynamic equihbtium. Chemical modification of the zeohte causing the pore diameter to be reduced produces catalysts that achieve almost 100% selectivity to -xylene. This favorable result is explained by the greatly reduced diffusivity of 0- and / -xylene compared with that of the less bulky -isomer. For the same reason, large crystals (3 llm) of ZSM-5 produce a higher ratio of -xyleneitotal xylenes than smaller crystahites (28,57). 

The thermodynamic equilibria are illustrated in Figures 1 and 2. Figure 1 shows the resulting composition after pure pseudocumene or a recycle mixture of C PMBs is disproportionated with a strong Friedel-Crafts catalyst. At 127°C (400 K), the reactor effluent contains approximately 3% toluene, 21% xylenes, 44% C PMBs, 29% C q PMBs, and 3% pentamethylbenzene. The equihbrium composition of the 44% C PMB isomers is shown in Figure 2. Based on the values at 127°C, the distribution is 29.5% mesitylene, 66.0% pseudocumene, and 4.5% hemimellitene (Fig. 2). After separating mesitylene and hemimellitene by fractionation, toluene, xylenes, pseudocumene (recycle plus fresh), C q PMBs, and pentamethylbenzene are recycled to extinction. 

This decomposition is thermodynamically favored by decreasing temperature and increasing pressure (28). Decomposition is extremely slow below 673 K in the absence of a catalyst however, between 673—873 K many surfaces, particularly iron (29), cobalt, and nickel (30), promote the disproportionation reaction. 

The coordination chemistry of NO is often compared to that of CO but, whereas carbonyls are frequently prepared by reactions involving CO at high pressures and temperatures, this route is less viable for nitrosyls because of the thermodynamic instability of NO and its propensity to disproportionate or decompose under such conditions (p. 446). Nitrosyl complexes can sometimes be made by transformations involving pre-existing NO complexes, e.g. by ligand replacement, oxidative addition, reductive elimination or condensation reactions (reductive, thermal or photolytic). Typical examples are ... 

Purification is by low-pressure distillation. Some physical properties are in Table 15.12. Polysulfanes are readily oxidized and all are thermodynamically unstable with respect to disproportionation ... 

SSF2 can be heated to 250° but is, in fact, thermodynamically unstable with respect to disproportionation, being immediately transformed to SF4 in the presence of acid catalysts such as BF3 or HF ... 

Anhydrous NaC102 crystallizes from aqueous solutions above 37.4° but below this temperature the trihydrate is obtained. The commercial product contains about 80% NaC102. The anhydrous salt forms colourless deliquescent crystals which decompose when heated to 175-200° the reaction is predominantly a disproportionation to C103 and Cl but about 5% of molecular O2 is also released (based on the C102 consumed). Neutral and alkaline aqueous solutions of NaC102 are stable at room temperature (despite their thermodynamic instability towards disproportionation as evidenced by the reduction potentials on p. 854). This is a kinetic activation-energy effect and, when the solutions are heated near to boiling, slow disproportionation occurs ... 

The modes of thermal decomposition of the halates and their complex oxidation-reduction chemistry reflect the interplay of both thermodynamic and kinetic factors. On the one hand, thermodynamically feasible reactions may be sluggish, whilst, on the other, traces of catalyst may radically alter the course of the reaction. In general, for a given cation, thermal stability decreases in the sequence iodate > chlorate > bromate, but the mode and ease of decomposition can be substantially modified. For example, alkali metal chlorates decompose by disproportionation when fused ... 

For bromates and iodates, disproportionation to halide and perhalate is not thermodynamically feasible and decomposition occurs either with formation of halide and liberation of O2 (as in the catalysed decomposition of CIOs just considered), or by formation of the oxide ... 

For all three halates (in the absence of disproportionation) the preferred mode of decomposition depends, again, on both thermodynamic and kinetic considerations. Oxide formation tends to be favoured by the presence of a strongly polarizing cation (e.g. magnesium, transition-metal and lanthanide halates), whereas halide formation is observed for alkali-metal, alkaline- earth and silver halates. 

These values should be compared with those for the other halogens (in 1 M acid) (p. 854). Noteworthy features are that At is the only halogen with an oxidation state between 0 and V that is thermodynamically stable towards disproportionation, and that the smooth trends in the values of E°( X2/X ) and (HOX/jXi) continue to At. 

The synthetic utility of the alkene metathesis reaction may in some cases be limited because of the formation of a mixture of products. The steps of the catalytic cycle are equilibrium processes, with the yields being determined by the thermodynamic equilibrium. The metathesis process generally tends to give complex mixtures of products. For example, pent-2-ene 8 disproportionates to give, at equilibrium, a statistical mixture of but-2-enes, pent-2-enes and hex-3-enes ... 

For example, disproportionation of but-2-yl radicals produces a mixture of butenes as shown (Scheme 1.1 I).138 Thermodynamic considerations suggest thai but-l-ene and but-2-enes should be formed in a ratio of ca 2 98. However, the observed 5 4 ratio of but-1-ene but-2-enes is little different from the 3 2 ratio that is expected on statistical grounds (i.e. ratio of f5-hydrogens in the I- and 3-positions). 

With the radical 29, even though loss of an equatorial hydrogen should be sterically less hindered and is favored thermodynamically (by relief of 1,3 interactions of the axial methyl), there is an 8-fold preference for loss of the axial hydrogen (at 100 ( i. The selectivity observed in the disproportionation of this and other substituted cyclohexyl radicals led Beckwith18 to propose that disproportionation is subject to stereoelectronic control which results in preferential breaking of the C-H bond which has best overlap with the orbital bearing the unpaired spin. 

ESI mass spectrometry ive mass spectrometry ESR spectroscopy set EPR spectroscopy ethyl acetate, chain transfer to 295 ethyl acrylate (EA) polymerizalion, transfer constants, to macromonomers 307 ethyl methacrylate (EMA) polymerization combination v.v disproportionation 255, 262 kinetic parameters 219 tacticity, solvent effects 428 thermodynamics 215 ethyl radicals... 

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.)... 

When the reaction conditions approach the thermodynamic equilibrium, isomerization follows. The distribution of the double bond is statistical. The molecular formation in the disproportionation stage is also statistical. Normally a run will produce 10-15% by weight of product, which is then suitable for LAB synthesis after distillation. The physical data of these internal olefins are shown in Table 4 [41]. 

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 chemistry of [Rh(OEP)h in benzene is dominated by Rh—Rh bond homolysis to give the reactive Rh(Il) radical Rh(OEP)-. This contrasts with the reactivity of fRh(OEP)] in pyridine, which promotes disproportionation via the formation of the thermodynamically favorable Rh(IlI). ct complex [RhjOEPKpy) ] together with the Rh(l) anion, Rh(OEP)J The hydride complex Rh(OEP)H shows NMR chemical shift changes in pyridine consistent with coordination of pyridine, forming Rh(OEP)H(py). Overall, solutions of Rh(OEP)l in pyridine behave as an equimolar mixture of [Rh(OEP)(py ) and (Rh(OEP). For example, reaction... 

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... 

This order of the potentials indicates that arsenic(IV) is an intermediate in the oxidation of arsenicflll) and reduction of arsenic(V) which is unstable from thermodynamic point of view and disproportionates easily according to... 

E° and E2° values of +76 and +21 mV, respectively, have been measured for Hox from M. trichosporium OB3b by similar methods (63). These values are more closely spaced and imply that Hmv from this organism is thermodynamically less stable with respect to disproportionation. Addition of protein B lowered the potentials to -52 mV and -115 mV, respectively. The regulation of electron transfer to the hydroxylase with protein B and reductase observed with the M. capsulatus (Bath) MMO was not seen with this system. Instead, it was reported that the potentials of Hox and of Hox with added protein B are shifted slightly to more positive values in the presence of reductase (Table II), and the reduction was not substrate-dependent. 

Calculations have estimated that decomposition of [TIMe] by either disproportionation to T1 and TlMe3 or by reductive coupling to give T1 and C2H6 are thermodynamically favored. See ... 

Unsually short NMR T, relaxation values were observed for the metal-bonded H-ligands in HCo(dppe)2, [Co(H2)(dppe)]+ (dppe = l,2-bis(diphenylphosphino)ethane), and CoH(CO) (PPh3)3.176 A theoretical analysis incorporating proton-meta) dipole-dipole interactions was able to reproduce these 7) values if an rCo H distance of 1.5 A was present, a value consistent with X-ray crystallographic experiments. A detailed structural and thermodynamic study of the complexes [H2Co(dppe)2]+, HCo(dppe)2, [HCo(dppe)2(MeCN)]+, and [Co(dppe)2(MeCN)]2+ has been reported.177 Equilibrium and electrochemical measurements enabled a thorough thermodynamic description of the system. Disproportionation of divalent [HCo(dppe)2]+ to [Co(dppe)2]+ and [H2Co(dppe)2]+ was examined as well as the reaction of [Co(dppe)2]+ with H2. 

Special synthetic routes are necessary to gain access to the metastable monohalides of A1 and Ga, since they are thermodynamically unstable at room temperature toward disproportionation into A1 metal and A1X3 or Ga metal and GaX3. First, the gaseous MX species, that are thermodynamically stable at high temperatures,22 have to be synthesized. A variety of methods are available for their preparation, some of which are shown in equations (a)-(c) of Scheme 2. 

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