Oxidation of Aldehydes to Acids

Rates of oxidation of benzaldehyde and of its 4-nitro derivative by pyridinium dichromate in aqueous acetic acid are first-order with respect to substrate and oxidant, and second-order in 

Kinetics of the oxidation of crotonaldehyde by tetraethylammonium chlorochromate have been measured in 50/50 aqueous acetic acid, including derivation of activation parameters.  

The Keggin-type phosphotungstic acid has been used to catalyse oxidation of substituted benzaldehydes by N-bromophthalimide in aqueous acetic acid, using mercury(II) as a scavenger. i Reaction rates are first-order in oxidant but fractional in aldehyde and phosphotungstate. 

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At the present time, the greatest importance of covalent hydration in biology seems to lie in the direction of understanding the action of enzymes. In this connection, the enzyme known as xanthine oxidase has been extensively investigated.This enzyme catalyzes the oxidation of aldehydes to acids, purines to hydroxypurines, and pteridines to hydroxypteridines. The only structural feature which these three substituents have in common is a secondary alcoholic group present in the covalently hydrated forms. Therefore it was logical to conceive of this group as the point of attack by the enzyme. 

Aldehyde dehydrogenase (EC 1.2.1.3) catalyzes the oxidation of aldehydes to acids (see Sect. 3.7.2). The enzyme is ubiquitously distributed, but has mainly been characterized in brain and liver, where it is found in the cytoplasm, mitochondria, and microsomes. It is not clear whether its esterase activity has a physiological role or is a surviving activity inherited from an evolutionary thiolesterase precursor. 

The slow spontaneous oxidation of compounds in the presence of oxygen is termed autoxidation (autooxidation). This radical process is responsible for a variety of transformations, such as the drying of paints and varnishes, the development of rancidity in foodstuff fats and oils, the perishing of rabber, air oxidation of aldehydes to acids, and the formation of peroxides in ethers. 

The oxidation of aldehydes to acids might follow the following mechanism ... 

Mechanisms of Aldehyde Oxidation. There must be at least two paths for oxidation of aldehyde to acid, and at least one of these must be temperature dependent. One pathway is the ozone-oxygen oxidation of aldehydes to peracids (14). However, peracid can also serve as an oxidizing agent for aldehyde. In the oxidation of acetaldehyde, Reaction 3 is thought to occur (14). 

The paper also reports two examples of oxidation of aldehydes to acids under these... 

Nonenzymlc autoxldatlon reactions which are predominant In highly unsaturated fish lipids, can be directed by the use of tocopherol-type antioxidants to manipulate oxidized flavors and thus Influence the quality of fish aromas and flavors. Secondary oxidation of aldehydes to acids by peraclds Is responsible for the formation of short chain fatty acids (C4 to Cg). These acids do not appear to contribute characterizing flavors and aromas In oxidizing fish lipids. 

Alcohol dehydrogenase catalyzes the oxidation of primary alcohols to aldehydes, as well as the reverse reaction. However, due to the facile oxidation of aldehydes to acids, the reaction equilibrium is driven to the right [Eq. (15)]. 

The unusual oxidant nickel peroxide converts aromatic aldehydes into carboxylic acids at 30-60 °C after 1.5-3 h in 58-100% yields [934. The oxidation of aldehydes to acids by pure ruthenium tetroxide results in very low yields [940. On the contrary, potassium ruthenate, prepared in situ from ruthenium trichloride and potassium persulfate in water and used in catalytic amounts, leads to a 99% yield of m-nitrobenzoic acid at room temperature after 2 h. Another oxidant, iodosobenzene in the presence of tris(triphenylphosphine)ruthenium dichloride, converts benzaldehyde into benzoic acid in 96% yield at room temperature [785]. The same reaction with a 91% yield is accomplished by treatment of benzaldehyde with osmium tetroxide as a catalyst and cumene hydroperoxide as a reoxidant [1163]. 

Direct anodic oxidation of aldehydes to acids has long been known, but has been largely neglected in recent years due to its limited preparative value [156,157]. Likewise, the anodic oxidation of ketones leads mainly to C-C bond cleavage reactions or to Ritter-type products when conducted in acetonitrile, though the reaction in the latter case is of some mechanistic interest [158-160]. 

Oxidation of carbonyl compounds. Oxidation of aldehydes to acids with Oxone ... 

For oxidation of aldehydes to acids (rather than esters), see Silver (II) oxide, this volume. 

The enzyme also catalyzes the oxidation of aldehydes to acids irreversibly (65). With catalytic amounts of NAD complete dismutation to equivalent amounts of acid and alcohol occurs. The mechanism has been studied in some detail (66). The < 2 (and K ) values for aldehydes in the aldehyde dehydrogenase reaction, measured by proton production, are large, comparable to those for secondary alcohols, as would be expected if the hydrated forms of the aldehydes are the true substrates... 

The initial complex may be in the form of the four-membered cyclic system called molozonide this may lead, in the presence of excess aldehyde, to the oxidation of aldehyde to acids. The reaction sequence is thus as follows ... 

Formation of an aldehyde such as 17 in the presence of a powerful oxidizing agent such as chromium(VI) is usually followed by rapid oxidation of 17 to the corresponding carboxylic acid, pentanoic acid (18). In other words, Jones s oxidation of simple aldehydes usually gives the carboxylic acid as the major product. If the reaction mixture is heated, overoxidation to the carboxylic acid is even more rapid. The fact that the Jones reagent rate of oxidation of alcohol to aldehyde is fast can be used to an advantage, and when acetone is used as a solvent, the rate of oxidation of aldehyde to acid is relatively slow. Acetic acid (ethanoic acid) serves a similar role in many oxidations. 

Gold nanoparticles supported on nanocrystalline cerium oxide, are extremely effective in the oxidation of aldehydes to acids and also in the oxidation of alcohols (see Vivier and Duprez and references therein). ... 

The oxidation of aldehyde to acid proceeds through a radical mechanism, and the metal complex does not play any significant role. However, the selective formation of ester in some of these reactions, e.g., with a ligand such as 8.19, is not by the reaction of the acid with the alcohol. Rather, reaction 8.5.1.2 is believed to be the mechanism behind selective ester formation. 

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