Reactivity disproportionation reactions

The behavior of oligosiloxanediols in the presence of strong bases is different. The contribution to the overall process of the disproportionation reaction, involving a migration of the ultimate siloxane unit between siloxane molecules, is much greater and may even completely dominate the polycondensation reaction (80). The reactivity enhancement of the siloxane bond adjacent to the sHanolate anion can be understood in terms of n(0) (7 (SiO) conjugation. 

Several side reactions or post-cuting reactions are possible. Disproportionation reactions involving terminal hydride groups have been reported (169). Excess SiH may undergo hydrolysis and further reaction between silanols can occur (170—172). Isomerization of a terminal olefin to a less reactive internal olefin has been noted (169). Viaylsilane/hydride interchange reactions have been observed (165). 

Chain reactions do not continue indefinitely, but in the nature of the reactivity of the free radical or ionic centre they are likely to react readily in ways that will destroy the reactivity. For example, in radical polymerisations two growing molecules may combine to extinguish both radical centres with formation of a chemical bond. Alternatively they may react in a disproportionation reaction to generate end groups in two molecules, one of which is unsaturated. Lastly, active centres may find other molecules to react with, such as solvent or impurity, and in this way the active centre is destroyed and the polymer molecule ceases to grow. 

The dinitrobenzenes display the characteristics of inhibitors for the more reactive vinyl acetate chain radicals. Two radicals are terminated during the induction period by each molecule of dinitrobenzene, indicating disappearance of inhibitor radicals by a disproportionation reaction. 

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]. 

Literature reports of NO disproportionation reactions with Fe(II) porphyrins contain many mutually inconsistent observations. Although facile NO disproportionation is promoted by Ru(II) and Os(II) (88) porphyrins to yield N20 and the respective M(Por)(NO)(ONO) complexes, the reactivity appears to be quite different with analogous Fe(II) complexes. Ferrous porphyrins such as Fen(TPP) undergo NO addition in ambient temperature solution to give the relatively stable... 

Table VI shows the relative reactivities of various asym DAMs. An equi-molecular mixture of two kinds of asym DAMs was fed as a 5% benzene solution and hydrogenolyzed in order to check the effect of the methyl group on the reactivity. Two kinds of asym DAMs having similar reactivities were selected as a combination. The reaction conditions were temperature, 400°C H2/hydrocarbons molar ratio, 2. The contact time was changed since the reactivities of asym DAMs differed considerably according to their structures this made it possible to evaluate the different reactivities. Side reactions such as demethylation, isomerization, and disproportionation were negligible under these reaction conditions. The relative values for the reactivities of the asym DAMs shown in Table VI are determined when the value of 2,5-DMeDPM as a standard material is fixed at 100.

As compared with the hydrolytic polymerization, the base catalysed polymerization is essentially more sensitive towards impurities. The inhibition or retardation by traces of various foreign substances follows from the extremely low concentrations of the imide formed by the disproportionation reaction ranging about in the order of magnitude 10 2mol-%, and from the high reactivity of the imide. The imides are the key intermediate in the polymerization mechanism and hence every substance which is able to react with imides acts either as an inhibitor or as a retarder of the base catalysed polymerization of caprolactam (67). 

Catalytic reduction of aromatic nitro compounds to the amines is highly exothermic (AH = —548 12 kJ/mol) and has high potential for hazard in the event of cooling- or other process-failure. The total reaction proceeds via nitroso and hydroxylamino intermediates, both of which are reactive and may undergo undesired condensation or disproportionation reactions, and the thermochemistry of all these possibilities was investigated. The reduction or disproportionation of the hydroxylamino intermediate (which is of low thermal stability) is identified as the fastest and most exothermic step (despite which it can frequently be concentrated or trapped) implications for process safety are considered in detail and verified by experiment with typical compounds and intermediates [1]. A calorimetric study of the hazards inherent in hydrogenation of nitroaromatics was made, using nitrobenzene as model compound [2]. Individual incidents of this type are ... 

The reactivity of i satin derivatives towards o/T/io-aminophenol and ortho-aminothiophenol has been the subject of a number of reports and some of the products obtained are quite intriguing. The first report attests that 1-acetylisatin reacts with o-aminophenol to furnish a ring opened product in ethanol as well as in AcOH. The same result occurred with o-aminothiophenol in acetic acid, whilst in ethanol two different products were formed in a disproportionation reaction, as can be inferred from the change of the oxidation state of what was the 1-acetyli satin C-3 ketone group. The structures were assigned based upon spectroscopic data and, for the benzothiazole derivative, on comparison with a sample... 

Disproportionation reaction rates depend on carbenium ion reactivities, which are determined by catalyst site acid strength. Carbenium ions produced at strong acid sites are less likely to undergo P-scission or desorption. Compared with HY, the smaller pores in HZSM-5 inhibit bimolecular disproportionation reactions. In contrast, the low paraf-fin/olefin volatile product ratio for the PE-MCM-41 sample is likely due to the low acidity of the catalyst. Although the MCM-41 pore size is large enough to facilitate disproportionation, catalytic site acidity is too low for this reaction pathway to be dominant. 

Encounters between silyl radicals in solution or in the gas phase usually result in recombination and disproportionation (45, 46). Disproportionation results in the production of silanes and highly reactive silenes. The disproportionation reaction is thermodynamically favorable because of the formation of a silicon-carbon double bond, which, although subsequently chemically reactive, is worth —39 kcal/mol (44). For pentamethyldisilanyl radicals, disproportionation is kinetically competitive with radical dimerization (46). In an earlier study, Boudjouk and co-workers (47) demonstrated conclusively by isotopic substitution and trapping that the silyl radicals generated by photolysis undergo disproportionation, as well as, presumably, dimerization (Scheme I). In deuterated methanol, the silanes produced were predominantly undeuterated, whereas methoxymethyldiphenylsilane was extensively deuterated in the a position. The results of these experiments strongly implicated the substituted silene produced by disproportionation. 

Electrochemical techniques are a convenient means of studying one-electron oxidations of amines. The reaction pattern of the anodic oxidation of amines depends greatly on the reaction conditions, including the nature of the electrode and the nucleophilicity of the solvent [1-3]. A major drawback of electrode oxidations is that unwanted secondary electron-transfer reactions can occur at the electrode surface. Also in electrochemical processes the effective reaction volume is limited at the electrode surface, thereby creating a high local concentration of reactive intermediates which can lead to dimerization and disproportionation reactions. These factors have to some extent, limited the synthetic utility of the anodic oxidation of amines. Because of this the anodic oxidation of amines has been intensively studied, although mainly from a mechanistic standpoint. 

The initiation reaction involves formation of acyllactam growth centres of at least the same reactivity as the growing end of the polymer molecule. The growth centres are formed either in the slow base catalysed disproportionation reactions (23) and (26) or in the reaction of the activator with the lactam or its anion, e.g. 

Thus, we must consider those reactions (of termination of macroradical with primary radical (Eq. (9)) which have not necessarily the same reactivity ratio, as the termination of two macroradicals. And this makes it often possible to obtain telechelic oligomers for monomers which give a non-negligible amount of disproportionation reactions in traditional polymerization. 

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