Disproportionation equilibrium reactions

In the following, the MO applications will be demonstrated with two selected equilibrium reactions, most important in radical chemistry disproportionation and dimerization. The examples presented will concern MO approaches of different levels of sophistication ab initio calculations with the evaluation of partition functions, semiempirical treatments, and simple procedures employing the HMO method or perturbation theory. 

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. 

Disproportionation is also an important method of forming polysilanes. The method is used on an industrial scale. It is important that disilanes are much easier to disproportionate than monosilanes. The reason seems to be the strong nucleophilic reactivity of the SiSi bond. It has been shown that different substituents exhibit different reactivities and this reactivity sequence is the one that we have found in equilibrium reactions. Investigations of several substituents show a series with a decreasing exchange rate valid for equilibrium and disproportionation reactions [16]. 

Although Hg has two oxidation states, there is a relatively small amount of redox chemistry associated with the group. Many reactions of Hg(I) and Hg(II) appear to involve the disproportionation equilibrium ... 

Thus, the copper ion is not an important species in aqueous solution, though copper does exist in solid compounds such as CuCl. In the presence of Cl- ions, the disproportionation equilibrium is reversed because precipitation of the insoluble, white copper chloride drives the following reaction to the right ... 

Disproportionation mechanisms have been proposed for protonation reactions and intramolecular rearrangements (see Sec. III.B and III.D) [50,51]. Their 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 disproportionation equilibrium constant. 

The cation radical of thianthrene (LVII" ") has been subject of extensive investigation [3, 187]. Voltammetric experiments with thianthrene (LVII) when carried out in the presence of TEA and its anhydride or activated neutral alumina gave reversible oxidation potentials in a variety of solvents, and disproportionation equilibrium constants, Xd,sp, were calculated [190]. Accurate values of X isp are important in the mechanistic evaluation for the reaction of LVII" with nucleophiles and have been used to rule out the disproportionation pathway, such as the reaction with water [191]. 

An indication that the intramolecular redox equilibrium between the first polypyridine reduction and the more negative vinyl ligand reduction is responsible for the radical initiation and subsequent film formation reaction of Ru(bpy)2(vinyl-ligand)22+ complexes is demonstrated in Figure 7. From the data for the various Ru(bpy)2(4 -X-stilb)2 + complexes in Table 2 it is seen that the first bipyridine reductions (Ej red.l)) all occur at approximately the same potential (-l.22jj3.03V). Thus, a disproportionation equilibrium will exist (eq. 13) which depends upon the reduction potential of the coordinated vinyl ligand. 

A new and potentially economic route for the preparation of carbonyl difluoride has been found recently. This involves the catalytic disproportionation of carbonyl chloride fluoride over activated charcoal in a flow system at low temperatures (20-177 C) [8Sa,ICI110]. The high yields obtained in the process correspond closely to those calculated for the equilibrium reaction ... 

Synthesis of 1,6-diaminohexane from the corresponding diol and ammonia is complicated by cyclization either via the amino alcohol intermediate or by disproportionation of the diamine (Scheme 8) [24], The best selectivity was achieved with Raney Ni in dioxane - 67 % for the diamine and 33 % for hexamethylenei-mine at 58% conversion. The selectivity for 1,6-diaminohexane could be increased to nearly 100 % by partial recycling of the cyclic imine, i. e. no additional imine was formed in the equilibrium reaction. 

Radicals RO play an important role in the oxidation mechanism of hydrocarbons and carbon-chain polymers. It was proved that phenoxyls are formed from phenols by their action44 49 85,143 144) followed by formation of phenolic and quinone methionide compounds in coupling reaction and by disproportionation. Also the reaction between RO and In and formation of alkoxycyclohexadienones of the type CIX may be presumed. Such compounds have not been isolated from the reaction mixtures yet. However, their formation was proved by spectra145, 179) and they probably appear only temporarily during the inhibited oxidation due to their low stability. According to1441, the following equilibrium reaction takes place ... 

The foregoing discussion and the examples given show the important effects of solvents and counterions on redox equilibriums. The most dramatic effects are seen in the disproportionation equilibriums described by the following reactions ... 

Reaction of Sodium Salt of Naphthacene Dianions with Na-phthacene. An example of electron transfer that reveals the role of solvation in such a process is the reaction of the sodium salt of naphthacene dianions (Nap2-) with naphthacene (Nap) in benzene (15). To increase the solubility of this salt in benzene, small amounts of THF are added to the solution. Apparently, more molecules of THF solvate the Na+ cations associated with naphthacene radical anions (Nap -) than the Na+ cations aggregated with Nap2- dianions. Hence, the disproportionation equilibrium for this system is described by the following reaction ... 

This is also an equilibrium reaction which does not favor conversion to DPC unless DMC is distilled from the system. The catalyst for the disproportionation reaction are either salts or alkoxides of Zn and Ti [29]. 

The equilibrium constant for this reaction is very favourable in each case 10 for ClO , lO for BrO , and 10 ° for IO . However, particularly in the case of ClO , the rate of disproportionation is slow at room temperature and only becomes appreciable above 70°. Similarly, the disproportionation... 

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