Reaction hydrogen disproportionation

Neither substance catalyzes hydrogen disproportionation reaction of cycloolefins. An essential difference was found in the action of these catalysts on alcohols. In the presence of vanadia, alcohols are hydro-genolyzed to the corresponding paraffins. At comparable conditions in the presence of chromia, alcohols undergo a dehydrogenation-condensation reaction with production of ketones. 

This is a disproportionation reaction, and is strongly catalysed by light and by a wide variety of materials, including many metals (for example copper and iron) especially if these materials have a large surface area. Some of these can induce explosive decomposition. Pure hydrogen peroxide can be kept in glass vessels in the dark, or in stone jars or in vessels made of pure aluminium with a smooth surface. 

The production of sihcon tetrachloride by these methods was abandoned worldwide in the early 1980s. Industrial tetrachlorosilane derives from two processes associated with trichlorosilane, the direct reaction of hydrogen chloride on sihcon primarily produced as an intermediate for fumed sihca production, and as a by-product in the disproportionation reaction of trichlorosilane to silane utilized in microelectronics. Substantial quantities of tetrachlorosilane are produced as a by-product in the production of zirconium tetrachloride, but this source has decreased in the 1990s owing to reduction in demand for zirconium in nuclear facihties (see Nuclearreactors). The price of tetrachlorosilane varies between l/kg and 25/kg, depending on grade and container. 

Aldehydes 1 that have no a-hydrogen give the Cannizzaro reaction upon treatment with a strong base, e.g. an alkali hydroxide.In this disproportionation reaction one molecule is reduced to the corresponding alcohol 2, while a second one is oxidized to the carboxylic acid 3. With aldehydes that do have a-hydrogens, the aldol reaction takes place preferentially. 

Cobalt(II) complexes of three water-soluble porphyrins are catalysts for the controlled potential electrolytic reduction of H O to Hi in aqueous acid solution. The porphyrin complexes were either directly adsorbed on glassy carbon, or were deposited as films using a variety of methods. Reduction to [Co(Por) was followed by a nucleophilic reaction with water to give the hydride intermediate. Hydrogen production then occurs either by attack of H on Co(Por)H, or by a disproportionation reaction requiring two Co(Por)H units. Although the overall I easibility of this process was demonstrated, practical problems including the rate of electron transfer still need to be overcome. " " ... 

In order to document the radical disproportionation reaction, we have used FT-IR spectroscopy to characterize the irradiation products. Upon irradiation of 1 in pentane, the formation of the characteristic peak near 2100 cm-1 due to Si-H stretching vibrations was readily apparent. The IR spectrum obtained in perdeuterated pentane was identical, suggesting that radical processes other than abstraction from the solvent are involved. Furthermore the ESR spectrum obtained in this solvent is identical to that already described. This raises the question whether the initially formed silyl radicals really abstract hydrogen from carbon with the formation of carbon-based radicals as suggested (13), particularly in light of the endothermicity of such a process. 

The presence of /3-hydrogen in the nitroxide radical may lead to disproportionation reactions. In spin-trapping experiments, N-t-butyl-a-phenyl nitrone yields rather unstable spin adducts. This type of radical can be stabilized by coordination to Nin. The Ni11 complex with N-oxy-A-r-butyl-(2-pyridyl)phenylmethanamine (923) reveals a distorted octahedral geometry with antiferromagnetic interactions between the unpaired electrons of the metal ion and the radical spins.00... 

The alkyl radicals possess reducing activity as well. In the disproportionation reaction, one alkyl radical reacts as an acceptor and another as a donor of the hydrogen atom. This is the reason for the reducing action of alkyl radicals in reactions with peroxides. 

Formation of oil from kerogen is a disproportionation reaction related to a hydrogen-rich mobile phase and a hydrogen-depleted carbon residue. After crystalline... 

Termination can take place either by the combination of two free radical chains to form one molecule or by transfer of a hydrogen atom in a disproportionation reaction. 

Note that the disproportionation reaction involves intermolecu-lar hydrogen abstraction. Due to steric factors, tertiary alkyl radicals undergo disproportionation rather than radical combination. Radicals forming addition products with compounds containing C=C bonds ... 

Diimide can act as both a hydrogen acceptor and donor, undergoing disproportionation as a side-reaction which produces a considerable amount of nitrogen gas. From a practical point of view the occurrence of this disproportionation reaction requires the use of an excess of the diimide precursor. 

Disproportionation reaction 7 might be expected to be thermoneutral in the gas phase and perhaps less so in the liquid phase where there is the possibility of hydrogen-bonding. Only for gas phase dimethyl peroxide is the prediction true, where the reaction enthalpy is —0.2 kJmoD. The liquid phase enthalpy of reaction is the incredible —61.5 kJmoD. Of course, we have expressed some doubt about the accuracy of the enthalpy of formation of methyl hydroperoxide. For teri-butyl cumyl peroxide, the prediction for thermoneutrality is in error by about 6 kJmor in the gas phase and by ca 9 kJmoD for the liquid. The enthalpy of reaction deviation from prediction increases slightly for tert-butyl peroxide — 14kJmol for the gas phase, which is virtually the same result as in the liquid phase, — 19kJmol . The reaction enthalpy is calculated to be far from neutrality for 2-fert-butylperoxy-2-methylhex-5-en-3-yne. The enthalpies of reaction are —86.1 kJmoD (g) and —91.5 kJmol (Iq). This same species showed discrepant behavior for reaction 6. Nevertheless, still assuming thermoneutrality for conversion of diethyl peroxide to ethyl hydroperoxide in reaction 7, the derived enthalpies of formation for ethyl hydroperoxide are —206 kJmoD (Iq) and —164 kJmoD (g). The liquid phase estimated value for ethyl hydroperoxide is much more reasonable than the experimentally determined value and is consistent with the other n-alkyl hydroperoxide values, either derived or accurately determined experimentally. 

Dihydropyrimidines are normally readily oxidized to the corresponding pyrimidines by dehydrogenation, hydrogen transfer, or disproportionation reactions <1994HC(52)1, 1996CHEC-II(6)93>. For example, the oxidation of a series of trifluoromethyl ketones 522 with DDQ occurred readily at room temperature <1997H(44)349>. Facile room temperature oxidation with ceric ammonium nitrate (CAN) has also been achieved <2003ARK(xv)22>. 

The mechanism of the electron transfer from electron donor such as MV to producing molecular hydrogen and catalyzed by Au [187] or Pt clusters [188], and that of the disproportionation reaction of the superoxide radical anion O2 by Pt clusters in solution or supported on colloidal Ti02 particles [189], have been studied by pulse radiolysis. 

The determination of equilibrium (standard) potentials is rather problematic for several reasons for instance, hydrolysis and disproportionation reactions, the existence of a large number of structural forms (e.g. a-, fi-, y-, 5-Mn02), strong dependence on pH and ionic exchange processes, and the instability of the species in contact with water (e.g. Mn-metal-hydrogen evolution, Mn04 oxygen evolution however, these processes are rather slow). 

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