Acetaldehyde formation from ethanol

Some of the substances are of greater concern than the others due to its relative quantities or to its flavored characteristic [1]. As an example, ethanol is the major compound in the group of alcohols being responsible for the formation of various other substances, such as acetaldehyde, resulting from ethanol oxidation, and it is the most abundant of the carbonylic compounds in distilled beverages. For the same reason, acetic acid is the major compound within its group, the carboxylic acids. 

Fig. 7. Branch point between fermentation and respiration. At low pyruvate flux, the low of the Pdh complex for pyruvate results in oxidative decarboxylation to form acetyl CoA and NADH. The acetyl CoA can then can go into energy generation (via respiration) or fatty acid synthesis. At high glycolytic flux, pyruvate accumulates, and the higher of Pdc favors acetaldehyde formation and ethanol production. Accumulation of acetate can interfere with mitochondrial function. Pyk Pyruvate kinase Pdh pyruvate dehydrogenase Pdc pyruvate decarboxylase Aid (Dha) aldehyde dehydrogenase Adh alcohol dehydrogenase Acs acetyl CoA synthetase. (Taken from Postma et al. [169])...

Figure 23-2. The oxidation of alcohols by alcohol dehydrogenase results in the formation of metabolites that cause serious toxicities. Ethanol, a preferred substrate for ADH, is used in methanol or ethylene glycol poisoning to slow the rate of formafion of fhe toxic metabolites of these alcohols. Acetaldehyde formed from ethanol is oxidized rapidly by aldehyde dehydrogenase except in the presence of disuifiram.

Hydrogenation of Acetaldehyde. Acetaldehyde made from acetylene can be hydrogenated to ethanol with the aid of a supported nickel catalyst at 150°C (156). A large excess of hydrogen containing 0.3% of oxygen is recommended to reduce the formation of ethyl ether. Anhydrous ethanol has also been made by hydrogenating acetaldehyde over a copper-on-pumice catalyst (157). 

In a similar manner, ethanol can be oxidized by the dichromate ion to form a compound called acetaldehyde, CHaCHO. The molecular structure of acetaldehyde, which is similar to that of formaldehyde, is shown at the bottom in Figure 18-6. We see that the molecule is structurally similar to formaldehyde. The methyl group, —CH3, replaces one of the hydrogens of formaldehyde. The balanced equation for the formation of acetaldehyde from ethanol is... 

Infrared spectroscopy has also been employed to follow the formation of acetaldehyde and acetic acid on Pt during ethanol electro-oxidation. On the basal planes, acetaldehyde could be observed starting at about 0.4 V (vs. RHE), well before the onset of CO oxidation, while the onset of acetic acid formation closely follows CO2 formation [Chang et al., 1990 Xia et al., 1997]. This is readily explained by the fact that both CO oxidation and acetic acid formation require a common adsorbed co-reactant, OHads, whereas the formation of acetaldehyde from ethanol merely involves a relatively simple proton-electron transfer. 

The mechanism of ethanol oxidation is less well established, but it apparently involves two mechanistic pathways of approximately equal importance that lead to acetaldehyde and ethene as major intermediate species. Although in flow-reactor studies [45] acetaldehyde appears earlier in the reaction than does ethene, both species are assumed to form directly from ethanol. Studies of acetaldehyde oxidation [52] do not indicate any direct mechanism for the formation of ethene from acetaldehyde. 

Alcohol-related liver diseases are complex, and ethanol has been shown to interact with a large number of molecular targets. Ethanol can interfere with hepatic lipid metabolism in a number of ways and is known to induce both inflammation and necrosis in the liver. Ethanol increases the formation of superoxide by Kupffer cells thus implicating oxidative stress in ethanol-induced liver disease. Similarly prooxidants (reactive oxygen species) are produced in the hepatocytes by partial reactions in the action of CYP2E1, an ethanol-induced CYP isoform. The formation of protein adducts in the microtubules by acetaldehyde, the metabolic product formed from ethanol by alcohol dehydrogenase, plays a role in the impairment of VLDL secretion associated with ethanol. 

Kinetic results for the reduction of ethyl acetate and acetic acid on similarly prepared 5 wt% Cu/SiC>2 catalysts are shown in Figs. 12 and 13 (75). The experiments were performed at 570 K at various partial pressures of hydrogen and oxygenated compounds. Measurements were also made at temperatures from 500 to 580 K (Figs. 12 and 13). It was observed that ethyl acetate reacts to form only acetaldehyde and ethanol, in equilibrium with each other (75). Figure 12 shows the effects of reaction conditions on the formation of ethanol when ethyl acetate is converted on Cu/SiC>2. 

The biosynthesis of optically active TIQs, exemplified by the presence of ( )-salsolinol in human fluids (189b), requires some comment. This is particularly warranted since many investigators have concluded that the conversion of dopamine to TIQ 64b is the result of condensation of the amine with acetaldehyde originating directly from ethanol. As shown in Fig. 32, this reaction, when carried out in vitro, affords racemic salsolinol (64) (5a,10). Formation of optically active TIQ 64b from dopamine and acetaldehyde would require that the Pictet-Spengler reaction be enzymatically controlled, as observed in the condensation of dopamine with 4-hydroxyphenylacetaldehyde in benzylisoquinoline-producing plants (209). The enzyme required to perform this reaction in mammalian systems has not yet been found. There are several observations which dispute such a reaction taking place in mammals the finding of 1-carboxy-TIQ 91 and DIQ 69 as major metabolites (189) and the very low levels of acetaldehyde detected in the brains of animals after alcohol consumption (215,216). This makes the acetaldehyde route to optically active 1-methyl-substituted TIQ suspect. 

On the other hand, the absorption band due to a carbonyl group (at about 1725 cm ) displays, at high potentials, a lower intensity on PtSn (the absorption band located close to 1725 cm remains as a shoulder even at high potentials) than on Pt (Fig. 34b), which indicates that the formation of C2 species (presumably acetaldehyde) resulting from a non-dissociative adsorption of ethanol is lower on a PtSn catalyst. 

K 29 is the calibration constant for m/z = 29 determined from ethanol oxidation on a gold electrode, which present a current efficiency close to 90% for the formation of acetaldehyde. " " The authors also monitored the mass spectrometric signal for m/z = 15 and m/z = 30 corresponding to fragment of methane (CHs ) and ethane (C2He ), respectively. 

A diethyl ether cool flame, followed by a second-stage flame can be stabilized in a tube [74] or above a burner [75—77], and Agnew and Agnew [78] have used a quartz probe to remove samples from various positions in these flames. Numerous products were identified including not only carbon monoxide, carbon dioxide, water, various saturated and unsaturated hydrocarbons, acetaldehyde, formaldehyde, methanol, ethanol and acetic acid, but also ethyl formate, ethyl acetate, acetone, propionaldehyde and 2-methyl-l 3-dioxacyclopentane. The main features of the analytical results were... 

Enzyme Cofactors- In many enzymatic reactions, and in particular biological reactions, a second substrate (i.e., species) must be introduced to activate the enzyme. This substrate, which is referred to as a cofactor or coenzyme even though it is not an enzyme as such, attaches to the enzyme and is most often either reduced or oxidized during the course of die reaction. The enzyme-cofactor complex is referred to as a holoenzyme. The inactive form of the enzyme-cofactor complex for a specific reaction and reaction direction is called an apoenzyme. An example of the type of system in which a cofactor is used is the formation of ethanol from acetaldehyde in the presence of the enzyme alcohol dehydrogenase (ADH) and the cofactor nicotinamide adenine dinuoleotide (NAD) ... 

Catalytic activity of rare earth elements (i.e., lanthanides, symbol Ln) in homogeneous catalysis was mentioned as early as 1922 when CeCls was tested as a true catalyst for the preparation of diethylacetal from ethanol and acetaldehyde [1]. Solutions of inorganic Ln salts were subsequently reported to catalyze the hydrolysis of carbon and phosphorous acid esters [2], the decarboxylation of acids [3], and the formation of 4-substituted 2,6-dimethylpyrimidines from acetonitrile and secondary amines [4]. In the meantime, the efficiency of rare earth metals in heterogeneous catalysis, e. g., as promoters in lanthanide (element mixtures)-... 

A mixture of formalin and ethanol was passed at 240—320 C over various metal oxides supported on silica gel and metal phosphates. The main products were acrolein, acetaldehyde, methanol, and carbon dioxide. Acidic catalysts such as V-P oxides promoted the dehydration of ethanol to ethene. The best catalytic performances for acrolein formation are obtained with nickel phosphate and silica-supported tungsten, zinc, nickel, and magnesium oxides. With a catalyst with a P/Ni atomic ratio of 2/3, the yields of acrolein reach 52 and 65 mol% on ethanol basis with HCHO/ethanol molar ratios of 2 and 3, respectively. Acetaldehyde and methanol are formed by a hydrogen transfer reaction from ethanol to formaldehyde. Then acrolein is formed by an aldol condensation of formaldehyde with the produced acetaldehyde [40],... 

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