Acetaldehyde disproportionation

Abelsonite structure, 850 Abscesses imaging techniques radiopharmaceuticals, 992 Acetaldehyde disproportionation catalysts, 304 hydration... 

In contrast with the well-known Embden-Meyerhof-Pamass glycolysis pathway for the conversion of hexose sugars to alcohol, the steps in conversion of ethanol to acetic acid remain in some doubt. Likely, ethanol is first oxidized to acetaldehyde and water (39). For further oxidation, two alternative routes are proposed more likely, hydration of the acetaldehyde gives CH2CH(OH)2, which is oxidized to acetic acid. An alternative is the Cannizzaro-type disproportionation of two molecules of acetaldehyde to one molecule of ethanol and one molecule of acetic acid. Jicetobacter... 

The G-values of all products depend on the dose rate, not only that of acetaldehyde, the product of the chain reaction (reaction 53). At high dose-rates, the elimination of water (reaction 49) cannot compete effectively with the disproportionation and combination reactions of... 

In processes I and II, the acetyl and methyl radicals react with solvent molecules giving respectively acetaldehyde and methane. In the third process chain degradation occurs followed eventually by disproportionation. Evidently the same photolysis carried out in the presence of a second monomer (acrylonitrile yields graft copolymers (I and II) and, possibly, some block copolymers (III), together with homopolymer initiated by the methyl and acetyl radicals. The formation of graft copolymers seems more likely on account of the greater stability of the radicals produced in reaction schemes I and II compared to scheme III. 

The formation of 5,6-dihydro-2,4,6-trimethyl-l,3,5-dithiazine, 2,4,6-trimethyl-1,3,5-trithiane, and 3,5-d ime thy1-1,2,4-trithiolane by heating of acetaldehyde, hydrogen sulfide, and ammonia was outlined by Takken and coworkers (36) and is summarized in Figure 4. Under oxidative conditions, dialkyltrithiolanes are formed at low pH there is conversion to trialkyltrithianes at elevated temperature isomerization into trisulfides occurs, which compounds disproportionate into di and tetrasulfides and in the presence of ammonia, dithiazines are formed. These compounds and the conditions for their formation are of extreme importance for the production of desirable meat flavors. 

Although known as an explosive since 1894, PETN was used very little until after World War I when the ingredients to make the starting material became commercially available. The symmetrical, solid alcohol starting material, pentaerythritol, is made from acetaldehyde and formaldehyde, which react by aldol condensation under basic catalysis followed by a crossed Cannizzaro disproportionation to produce the alcohol and formate salt. Although the reaction takes place in a single mixture, it is shown below in two steps for clarity. 

Compound 79 is structurally related to TIQ 80, obtained on condensation of norepinephrine with formaldehyde (164), and to TIQ 81, detected in animal tissues after exposure to acetaldehyde (165). Acid-catalyzed dehydration of TIQ 82, the N-methyl analog of TIQ 79, should lead to the iminium species 83 (166), which on two-electron oxidation or by disproportionation should give the isoquinolinium salt 84. Such reactions, if occurring in vivo, would parallel similar reactions seen with the neurotoxin MPTP in its conversions to MPDP and MPP (167) and could possibly explain the neurotoxic effects seen with 117 (168). 

At about 500 °K disproportionation is favoured as the products are mainly ethanol and acetaldehyde. Explosion leads to higher temperatures, and more ethoxy radicals decompose yielding more ethane and formaldehyde. 

Dodd , as well as Ausloos and Steacie applied relation (35) to the experimental results of acetaldehyde photolysis. The data obtained at high temperatures seem to fit the Arrhenius straight line derived from the azomethane-acetaldehyde and di-/-butyl peroxide-acetaldehyde systems. At low temperatures, however, considerable deviation from this line could be observed, the possible result of additional methane producing reactions of some sort. According to Dodd, these processes could be (i) primary process II, (//) disproportionation reaction (26) and (in) the wall reaction of the methyl radicals. 

Alcohol is subject to first-order kinetics with a t) of about one hour at plasma concentrations below 10 mg/dl [attained after drinking about two-thirds of a unit (glass) of wine or beer]. Above this concentration the main enzyme (alcohol dehydrogenase) that converts the alcohol into acetaldehyde approaches and then reaches saturation, at which point alcohol metabolism caimot proceed any faster. Thus if the subject continues to drink, the blood alcohol concentration rises disproportionately, for the rate of metabolism remains the same (at about 10 ml or 8 g/h for a 70 kg man), i.e. a constant amount is metabolised in unit time, and alcohol shows zero-order kinetics. 

The CH8CH20 radicals disproportionate to give acetaldehyde and ethanol. 

The disproportionation leads to aldehyde and ethanol (Eq. 9.27a) whereas dimerization leads to butanediol (Eq. 9.27b and Fig. 9.25a). These products have indeed been found (concentration ratio of acetaldehyde to butanediol, 2.5) during illumination of a solution of 3-nm colloid [62]. Besides butanediol and acetaldehyde H2 was also detected, the latter being formed in the corresponding cathodic reaction. When the same experiment was performed with much larger particles (4 pm), no butanediol was found [62]. Since the time interval between the absorption events of two photons in one 4-pm particle is only 20 s, the radical can be oxidized to acetaldehyde by a further hole transfer at the same particle as illustrated in Fig. 9.25b [5]. Instead of Eq. (9.27) we have then... 

Ethyl acetate can be obtained by the dehydrogenation of ethanol in 95.7% yield.25 The process may involve a disproportionation of the intermediate acetaldehyde. It can also be made by direct esterification of ethanol using acetic acid made by fermentation. Acetone was prepared in 97-98% yield by treatment of acetic acid with cerium (IV) oxide on silica26 or manganese nodules from the Indian Ocean.27 This is an alternative to the fermentation that produces it along with 1-butanol and might be preferable, in that any process that produces two coproducts must find adequate outlets in the market for both of them. There is a loss of one carbon when acetic acid is converted to acetone. 

According to Sheldon et al. (1986), two molecules of the 2,3-dihydropyrazines can form a mixture of the corresponding pyrazine and 1,2,3,4-tetrahydropyrazine by disproportionation. It was also observed by Masuda et al. (1980) that dehydrogenation of 2,3-dimethyl-5,6-dihydropyrazine generated the disproportionation compounds 2,3-dimethyl-1,2,5,6-tetrahydropyrazine and 5-ethyl-2,3-dimethylpyrazine in addition to the desired 2,3-dimethylpyrazine in a sodium ethoxide/ethanol solution. It was then deduced that the carbanion of 2,3-dimethyl-5,6-dihydropyrazine was formed with the base and then reacted with acetaldehyde, present in ethanol in small quantities, to yield the 5-substituted pyrazine. On the basis of this result the authors prepared in high yield a series of nine, 5-substituted, 2,3-dimethylpyr-azines by reaction of 2,3-dimethyl-5,6-dihydropyrazines with six aldehydes and three ketones under the same basic conditions. 

The oxidation of ethanol was studied in detail by Zaikovetal. [3—7]. The main products of the oxidation in a steel autoclave under a pressure of 50—95 atm at 145—230°C are acetic acid and ethyl acetate with hydrogen peroxide and acetaldehyde as intermediates. Formic acid and methyl formate are produced in small amounts. The oxidation of ethanol proceeds with autocatalysis. Acetaldehyde is oxidized not only to acetic acid but also to ethyl acetate by disproportionation [8]... 

The disproportionation of cyclohexa-1,4-diene to benzene and cyclohexane is catalyzed by [Ir(X)(CO)(PPh3)2] (X = Cl, I), [Ir(Cl)(CO)(AsPh3)3] and [Ir(H)2(Cl)(CO)(PPh3)2]. The iridium(I) complex [Ir(cod)(MeCN)2] reportedly catalyzes both the disproportionation of cyclohexa-1,3-diene to benzene and cyclohexene, as well as the isomerization of cyclohexa-1,4-diene to the conjugated isomer.[Ir(Cl)2(C5Me5)]2 catalyzes the disproportionation of acetaldehyde to acetic... 

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