Oxidation of ethanol and acetaldehyde

Ethanol is oxidised to acetaldehyde, which in turn is oxidised to acetic acid. Perlmutter-Hayman and Weissmann showed that oxidation by molecular bromine predominates. Thus, for ethanol at pH 1.0 and 0 °C they found = 4 X 10 l.mole sec whereas kHBro about 30 times smaller, and for acetaldehyde under the same conditions, = 2.1 xlO l.mole .sec whereas HBro is about 50 times smaller. The reactions with hypobromous acid are much more light-sensitive than those with molecular bromine. 

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Lai SCS, Koper MTM. 2009. Electro-oxidation of ethanol and acetaldehyde on platinum single-crystal electrodes. Faraday Discuss 140 399-416. 

Sauer ML, Ollis DF (1996) Photocatalyzed Oxidation of Ethanol and Acetaldehyde in Humidified Air, J. Catal. 158 570-582. 

The work presented in this paper is the first part of a project aiming at the development of tailor-made oxidation catalysts for diesel engines fuelled by alcohol fuels, ethanol or methanol. The investigation is focused on the influence of support material on the low temperature oxidation of ethanol and acetaldehyde. The study presents results from an experimental investigation with precious metal catalysts applied on monolithic cordierite substrates. Platinum or palladium were applied onto a support consisting of either aluminum oxide, cerium dioxide, silicon dioxide or titanium dioxide. 

For the same purpose, a Pt/Ti02 catalyst was used in the deep oxidation of ethanol and acetaldehyde with molecular oxygen in SCCO2 at 150 to 300°C, but no significant improvement compared to the gas-phase process was achieved. ... 

There are two ways to produce acetaldehyde from ethanol oxidation and dehydrogenation. Oxidation of ethanol to acetaldehyde is carried out ia the vapor phase over a silver or copper catalyst (305). Conversion is slightly over 80% per pass at reaction temperatures of 450—500°C with air as an oxidant. Chloroplatinic acid selectively cataly2es the Uquid-phase oxidation of ethanol to acetaldehyde giving yields exceeding 95%. The reaction takes place ia the absence of free oxygen at 80°C and at atmospheric pressure (306). The kinetics of the vapor and Uquid-phase oxidation of ethanol have been described ia the Uterature (307,308). 

During World War II, production of butadiene (qv) from ethanol was of great importance. About 60% of the butadiene produced in the United States during that time was obtained by a two-step process utilizing a 3 1 mixture of ethanol and acetaldehyde at atmospheric pressure and a catalyst of tantalum oxide and siHca gel at 325—350°C (393—397). Extensive catalytic studies were reported (398—401) including a fluidized process (402). However, because of later developments in the manufacture of butadiene by the dehydrogenation of butane and butenes, and by naphtha cracking, the use of ethanol as a raw material for this purpose has all but disappeared. 

Formally, in redox reactions there is transfer of electrons from a donor (the reductant) to the acceptor (the oxidant), forming a redox couple or pair. Oxidations in biological systems are often reactions in which hydrogen is removed from a compound or in which oxygen is added to a compound. An example is the oxidation of ethanol to acetaldehyde and then to acetic acid where the oxidant is NAD. catalyzed by alcohol dehydrogenase and acetaldehyde dehydrogenase, respectively. 

Induced oxidation of alcohols by hydrogen peroxide was studied by Kolthoff and Medalia . According to their measurements the value of F-, increases with the increase in the concentration of ethanol, while it decreases with increase in the acid concentration (see Table 16). In acetic acid medium the value of F[ is considerably lower. Chloride ions effectively suppress the induced oxidation of alcohols. The main product of the oxidation of ethanol is acetaldehyde which can be further oxidized to acetic acid. The data on the induced oxidation of alcohol (H2A) can be interpreted by reactions (53), (98), (99) and (57). 

Figure 17.19 A membianeless ethanol/02 enz3fme fuel cell. Alcohol dehydrogenase and aldehyde dehydrogenase catalyze a stepwise oxidation of ethanol to acetaldehyde and then to acetate, passing electrons to the anode via the mediator NAD+/NADH. At the carhon cathode, electrons are passed via the [Ru(2,2 -bipyridyl)3] and biUverdin/bilimbin couples to bilirubin oxidase, which catalyzes O2 reduction to H2O. (a) Schematic representation of the reactions occruring. (b) Power/cmrent response for the ceU operating in buffered solution at pH 7.15, containing 1 mM ethanol and 1 mM NAD. Panel (b) reprinted from Topcagic and Minteer [2006]. Copyright Elsevier, 2006.

Both ADH and ALDH use NAD+ as cofactor in the oxidation of ethanol to acetaldehyde. The rate of alcohol metabolism is determined not only by the amount of ADH and ALDH2 enzyme in tissue and by their functional characteristics, but also by the concentrations of the cofactors NAD+ and NADH and of ethanol and acetaldehyde in the cellular compartments (i.e., cytosol and mitochondria). Environmental influences on elimination rate can occur through changes in the redox ratio of NAD+/NADH and through changes in hepatic blood flow. The equilib-... 

Also, using noble metals, the performances are still not satisfactory. There is not a total oxidation of ethanol and byproducts such as acetaldehyde are formed [48]. 

Nonporous silver membrane tube (99.99 wt.% Ag), (in double pipe configurationX thickness 100/im. Feed enters the reactor at shell side, oxygen at tube side. Oxidation of ammonia. Silver catalyst in membrane form (see previous column). Oxidation of ethanol to acetaldehyde. Silver catalyst in membrane form (see previous column). r- 250-380°C. The yield of nitrogen was 40%, the yield of nitrogen monoxide was 25%. r- 250-380°C. The yield of acetaldehyde was 83%. The yield with bulk powdered silver catalyst was 56%. Gryaznov, Vedernikov and Guryanova (1986)... 

NAD is one of Nature s most important oxidizing agents it can be considered as a biological equivalent of the chromium(VI) ion. NAD is shorthand for nicotinamide adenine dinucleotide it is a co-enzyme, which together with an enzyme is essential for several life-sustaining processes (Box 2.2). On reduction it forms the corresponding 1,4-dihydropyridine, NADH, The oxidation of ethanol to acetaldehyde (ethanal) is effected by the enzyme alcohol dehydrogenase and mediated by NAD (Scheme 2.31). 

Oxidation is another important factor for the wine-aging process. Major oxidation reactions taking place in wine following oxygen exposure actually involve other wine constituents that are primarily ethanol and, in the presence of metal ions, tartaric acid rather than flavonoids, although phenolic compounds have been shown to participate in oxidation of ethanol to acetaldehyde. ... 

Similarly, catechin polymers formed upon horseradish peroxidase-catalyzed oxidation of catechin or polycondensation of catechin with aldehydes prove much more efficient than catechin (at identical monomer concentration) at inhibiting XO and superoxide formation. A more detailed investigation with the catechin-acetaldehyde polycondensate (which is expected to form in wine because of the microbial oxidation of ethanol to acetaldehyde) shows that inhibition is noncompetitive to xanthine and likely occurs via binding to the FAD or Fe/S redox centers involved in electron transfers from the reduced molybdenum center to dioxygen with simultaneous production of superoxide. 

We shall start the discussion with a classical experiment related to the stereochemistry of oxidation of ethanol and reduction of acetaldehyde mediated by the enzyme yeast alcohol dehydrogenase in the presence of the oxidized (NAD+) and reduced (NADH) forms, respectively, of the coenzyme nicotinamide adenine dinucleotide (Fig. 54). The stereochemically interesting feature of this reaction stems from the fact that the methylene hydrogens in CH3CH2OH and the faces of the carbonyl in CH3CH = 0 are enantiotopic. The question thus arises which of the CH2-hydrogens... 

Fig. 55. Oxidation of ethanol and reduction of acetaldehyde by NAD+/NADH in the presence of yeast alcohol dehydrogenase (YADH)...

From the standard reduction potentials of half-reactions listed below, construct the plausible electron transfer system for the oxidation of ethanol to acetaldehyde and water. 

Oyama and Somorjai have studied the oxidation of ethanol and ethane over vanadia supported on silica.33 Ethane oxidation yielded ethylene and acetylene with considerable CO2 for highly dispersed catalysts. Ethanol oxidation to acetaldehyde, on the other hand was shown to be structure insensitive. Conversions and selectivities for reaction of ethanol are shown in Table III.33... 

Answer The first step in the synthesis of glucose from lactate in the liver is oxidation of the lactate to pyruvate like the oxidation of ethanol to acetaldehyde, this requires NAD+. Consumption of alcohol forces a competition for NAD+ between ethanol metabolism and gluconeogenesis, reducing the conversion of lactate to glucose and resulting in hypoglycemia. The problem is compounded by strenuous exercise and lack of food because at these times the level of blood glucose is already low. 

The peroxisome catalase system catalyses the hydrogen peroxide (H2O2 (-dependent oxidation of ethanol to acetaldehyde and water. Normally it contributes little to the oxidation of alcohol because of the limited availability of hydrogen peroxide. However, activation of peroxisomal catalase, by the increased generation of hydrogen peroxide via peroxisomal /3-oxidation, leads to an increased metabolism of alcohol. This state may contribute to an alcohol-related inflammation and necrosis in alcoholic liver disease. 

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