Acetaldehyde oxidation, mechanism

The theoretical explanation of the butane reaction mechanism is as fully developed as is that of acetaldehyde oxidation (51). The theory of the naphtha oxidation reaction is more troublesome, however, and less well understood. This is largely because of a back-biting reaction which leads to cycHc products (52). 

The present research was focused on the study of acetaldehyde oxidation rising air with aqueous mangan acetate catalyst in mechanically stirred tank reactor. 

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. 

It is not possible to propose a general mechanism from these studies, for results do not correspond to a definite pattern. Although, in all the systems, secondary amines are the most effective inhibitors, the role played by tertiary amines is confusing. In several systems (Table I, No. 1, 2, and 3) tertiary amines are much more effective than primary amines, but in others they appear to have little or no effect. Again, in acetaldehyde oxidation (Table I, No. 1 and 2) there is generally a linear relationship between the amount of inhibitor added and the induction period before either slow oxidation or ignition of the fuel occurs. In other systems (Table I, No. 3, 4, and 5), however, a much more complex relationship is obtained. Thus, amines may be acting by different mechanisms in different systems. 

A more complicated reaction scheme is proposed by the authors to include the formation of the by-products acetonitrile, acetaldehyde and ethylene. However, appropriate rate coefficients cannot be given as the reactions appear to be partially homogeneous gas phase reactions, implying that factors like the reactor geometry are also involved. Regarding the oxidation mechanism, the authors assume that two hydrogen atoms are first abstracted from propene, followed by reaction with surface oxygen or NH species. 

Peracetic Acid Decomposition. Although, by comparing the rate of peracetic acid decomposition with the rate of its reaction with acetaldehyde, we can rule out the decomposition as a major path in acetaldehyde oxidation (see below), we will discuss the possible mechanisms for the catalytic decomposition of peracetic acid. 

Aldehydes are considerably more reactive toward OH radicals than alkanes, so that the aldehydes produced are subject to further oxidation. Thereby additional NO is converted to N02. To give an example, let R, stand for CH3 so that RjCHO represents acetaldehyde. The mechanism for acetaldehyde oxidation may be written as follows ... 

The mechanism of acetaldehyde oxidation is relatively complex. Considering only molecular species, the main steps appear to be... 

Thus, many groups have sought alternative oxidants. A polyoxometaUate (POM) has been shown to act as a mediator of oxidation by 0 (Equation 18.9). In this case, the reaction of methane with O in the presence of Periana s catalyst supported on HjPVjMOjjO j as acid and mediator of oxidation has been reported to form a mixture of methanol and acetaldehyde. The mechanism of the formation of the acetaldehyde product from methane is not firm, but is proposed to occur by oxidative coupling of methane with formaldehyde, which would be generated from methanol. These reactions occur with modest turnover numbers of about 30, but the use of and a POM is a clear advance over the original Shilov process with platinum(IV) as the stoichiometric oxidant. 

The principal studies of acetaldehyde oxidation are those of Bogdanovskii and Shlygin, who have proposed an electron-radical mechanism which is of the form ... 

During heat treatment of foods, various aldehydes are formed in the head space. When pork fat was heated, various aldehydes such as formaldehyde, acetaldehyde, propanal, butanal, pentanal, and hexanal, were identified and quantitated (20). Hexanal was found in the largest amount, followed by pentanal and the other lower chain aldehydes. Both hexanid and pentanal have been commonly used as markers of lipid peroxidation in other food systems (27 j. However, irradiated cod liver oil produced malondialdehyde of which level was nearly five times higher than that of other aldehydes (22j, suggesting that different oxidative mechanisms are involved and that more Aan one spe c marker may be required to understand the formation of various aldehyde species. 

The system that has been subjected to the most detailed modern study is the oxidation of acetaldehyde at sub-atmospheric pressures in the temperature range 500-700 K, and it is to be discussed here in order to exemplify the Kinetic bacKground of oscillatory phenomena and the principal experimental observations. In chemical terms acetaldehyde oxidation is more simple than most organic oxidations yet it exhibits all of the thermoKinetic features so far discovered. It has been a natural choice for study not only because of the relative ease of understanding Kinetic mechanisms but also for convenience of experimental investigation. 

Oxidation mechanisms have been investigated for a number of larger aldehydes, using chamber methods with end product analysis. Tuazon et al. (2003) studied the mechanism of the reaction of OH radicals with 2,3-dimethylpentanal. They found that acetaldehyde and 2-butanone were the main products with a small yield (5.4%) of 3-methyl-2-pentanone. The latter product arises exclusively from abstraction at the 2-position, adjacent to the carbonyl group the equivalent abstraction is excluded from the butanal mechanism for the sake of clarity. 

Picquet-Varrault et al. (2002) have conducted a product study of the OH-radical-initiated oxidation of iso-butyl acetate in 1 atm. of air at 298 K in the presence of NOjt. Picquet-Varrault et al. (2002) reported the following products (molar yields) acetone (0.78 0.12), formic acetic anhydride (0.52 0.06), acetoxyacetaldehyde (0.18 0.06), acetic acid (0.08 0.02), acetaldehyde (<0.07), and acetoxyacetone (<0.02). As noted by Picquet-Varrault et al. (2002), because most of the observed products do not provide definite markers for the occurence of a specific reaction pathway, it is difficult to constract a precise oxidation mechanism. For example. 

Studies of the reaction mechanism of the catalytic oxidation suggest that a tit-hydroxyethylene—palladium 7t-complex is formed initially, followed by an intramolecular exchange of hydrogen and palladium to give a i yW-hydtoxyethylpalladium species that leads to acetaldehyde and metallic palladium (88-90). 

Facial dushing after ingestion of alcohol occurs in up to one-third of patients taking chlorpropamide. The mechanism, like that of the disulfiram reaction, probably involves inhibition of the oxidation of acetaldehyde, a metaboUte of ethanol. The plasma concentration of chlorpropamide may be correlated with chlorpropamide—alcohol dushing. 

Homogeneous Oxidation Catalysts. Cobalt(II) carboxylates, such as the oleate, acetate, and naphthenate, are used in the Hquid-phase oxidations of -xylene to terephthaUc acid, cyclohexane to adipic acid, acetaldehyde (qv) to acetic acid, and cumene (qv) to cumene hydroperoxide. These reactions each involve a free-radical mechanism that for the cyclohexane oxidation can be written as... 

Ethanol is oxidized by alcohol dehydrogenase (in the presence of nicotinamide adenine dinucleotide [NAD]) or the microsomal ethanol oxidizing system (MEOS) (in the presence of reduced nicotinamide adenine dinucleotide phosphate [NADPH]). Acetaldehyde, the first product in ethanol oxidation, is metabolized to acetic acid by aldehyde dehydrogenase in the presence of NAD. Acetic acid is broken down through the citric acid cycle to carbon dioxide (CO2) and water (H2O). Impairment of the metabolism of acetaldehyde to acetic acid is the major mechanism of action of disulfiram for the treatment of alcoholism. 

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