Oxidation of Alcohols to Aldehydes and Acids

Prior to 1916, acetaldehyde was manufactured by the oxidation of alcohol in the liquid phase with bichromate and sulfuric add.1 Since that time it has been ade quite largely by the hydration of acetylene in sulfuric acid solutions activated with mercury salts. However, the relatively low price of ethanol in America has made the formation of acetaldehyde by vapor phase dehydrogenation or limited oxidation of the alcohol attractive commercially. To this end several methods have been proposed for conducting the transformation industrially. Developments of processes employing vapor phase oxidation reactions have all been based largely on the prindples disclosed by the early work, a considerable portion of which had been undertaken purely for the purpose of research and not industrialization. 

In following the development of the process it is desirable to consider methanol oxidation separately since it presents problems that are unique. 

Consequently, alcohols of the aliphatic series other than methanol will be considered here. 

The form of copper used in tins work was such that the initial temperature of the reaction was much higher than when platinum was used. 

However, no data as to optimum temperatures or times of contact were given. Loew 8 in continuing the study of the oxidation of methanol confirmed Tollens in the feasibility of substituting copper for platinum as a catalyst and obtained somewhat higher yields of aldehydes. Kablukow 10 is also to be associated with Tollens and Loew in attempting to modify Hofmann s experiments in such a way as to facilitate the preparations of formaldehyde in relatively large quantities. 

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Activated charcoal, obtained from peat, lignite, anthracite coal, etc., has also been mentioned as a catalyst for the oxidation of alcohols to aldehydes and acids.79... 

It was found as early as the middle of the 19th century that oxidation of alcohols to aldehydes and acids with molecular oxygen is catalyzed by platinum metals, but these reactions have been comparatively little studied. After the pioneering work of Heyns et al. [1,2] most of the reports in the open literature were published by a small number of research groups at Eindhoven [3-24], Delft [25-39], Zurich [40-56], and Villeurbanne [57-68], and the oxidation of glycerol and derivatives was studied by Kimura [69-74]. A few review papers were devoted to the liquid-phase oxidation of alcohols and carbohydrates on metal catalysts [31,48, 75-77]. 

Oxidation Alcohols and Aldehydes. The oxidation of alcohols to aldehydes and of aldehydes to carboxylic acids is routine, and is catalyzed by alcohol dehydrogenase and aldehyde dehydrogenase, respectively. 

Oxidation of Alcohols. Conversion of alcohols to aldehydes and acids has received considerably less attention. The chemistry of this process has been explored primarily with respect to formation of stable hydrides of Pt, Ir, Ru, and Os (7, 8) (Reaction 4). 

Finely divided ruthenium acts as a catalyser, inducing, for example, the oxidation of alcohol to aldehyde and acetic acid in the presence of air. 

Oxidation of Alcohols to Aldehydes and Ketones with Chromic Acid Adsorbed on Silica Gel [5J7]... 

A homogeneous catalytic solution to the alcohol inhibition problem (see the discussion under Uncatalyzed chain reactions of the oxidation of alcohol intermediates, above) does not appear to have been found. However, the presence of a heterogeneous oxidative dehydrogenation catalyst has been reported to be effective in the direct oxidation of alcohols to carbonyls and acids [109, 110]. The mechanism probably involves preliminaiy heterogeneous (oxidative) dehydrogenation of carbinols to carbonyls. If the carbonyl is an aldehyde, it is readily converted to the acid. Platinum, palladium, ruthenium, rhodium, and iridium catalysts, supported on carbon, are reported to be active and selective catalysts for the purpose [109]. Promoters such as cobalt and cadmium have been reported to be effective additives. 

Cainelli, G., Cardillo, G., Orena, M., Sandri, S. Polymer supported reagents. Chromic acid on anion exchange resins. A simple and practical oxidation of alcohols to aldehydes and ketones. J. Am. Chem. Soc. 1976, 98, 6737-6738. 

Lei M, Hu RJ, Wang YG (2006) Mild and selective oxidation of alcohols to aldehydes and ketones using NalO /TEMPO/NaBr system under acidic conditions. Tetrahedron 62 8928-8932... 

Ru04] oxidizes alcohols and aldehydes to ketones and carboxylic acids, and readily cleaves carbon-carbon double bonds. " The use of Bu4N[Ru04], prepared by the fusion of RuClj with KNO3 and KOH and subsequent reaction with CI2 and addition of BU4NOH, for the oxidation of alcohols to aldehydes and ketones in acetone or CHjClj has been described. 

The second part of this text section introduces the oxidation of alcohols to aldehydes and ketones, the reverse of reduction. Alcohol oxidation is very useful in that it produces a carbonyl group, the most important functional group of all. Note the two types of reagents based on Cr(VI) PCC (pyridinium chlorochromate, pyH 1 CrO3Cl ), which is specifically intended for oxidation of primary alcohols to aldehydes, and aqueous dichromate, which oxidizes secondary alcohols to ketones, but overoxidizes 10 alcohols to carboxylic acids. 

In addition to the industrial apphcations, in Scheme 8.1, other reactions have been the focus of extensive research and development. For example. Chapter 12 surveys the research efforts directed toward Pd-catalyzed oxidative carbonylation of phenol affords the important monomer, diphenyl carbonate (Scheme 8.2a). Other reactions of potential industrial significance highlighted in this chapter include the oxidation of alcohols to aldehydes and ketones (Scheme 8.2b), oxidative coupling of arenes and carboxylic acids to afford aryl esters (Scheme 8.2c), benzylic acetoxylation (Scheme 8.2d), and oxidative Heck reactions (Scheme 8.2e). The chapter concludes by highlighting a number of newer research developments, including ligand-modulated catalytic oxidations, Pd/NO cocatalysis, and alkane oxidation. 

Oxidation of Alcohols to Aldehydes and Ketones. Hyper-valent iodine compounds have powerful oxidizing capabilities. However, IBX possesses different properties than many similar oxidants, such as the related analogs iodoxybenzene and m-iodoxybenzoic acid. Until recently, the major application of IBX was its use in DMSO for the oxidation of primary alcohols to aldehydes at room temperature, without the danger of over-oxidation to carboxylic acids. The related iodo-oxy reagents oxidize benzyl alcohols to benzaldehydes at elevated temperatures in benzene (80 °C, 5-10 h) or in acetic acid (rt, 24 h), while IBX oxidizes the same compounds in 15 min (or less) at room temperature. IBX is equally effective for the oxidation of secondary alcohols to ketones under analogous conditions. Even sterically hindered alcohols are readily oxidized. Bomeol undergoes smooth oxida-... 

It should come as no surprise that biological systems do not use agents such as potassium dichromate or the oxides of other transition metals for the oxidation of alcohols to aldehydes and ketones or for the oxidation of aldehydes to carboxylic acids. What biological systems use instead is NAD+ (AD = a combination of adenine and ribose).The function of the Ad portion of the molecule is to position NAD" on the surface of the enzyme in the proper orientation relative to the molecule it is to oxidize. 

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