Oxidations aldehydes

Reaction 21 is the decarbonylation of the intermediate acyl radical and is especially important at higher temperatures it is the source of much of the carbon monoxide produced in hydrocarbon oxidations. Reaction 22 is a bimolecular radical reaction analogous to reaction 13. In this case, acyloxy radicals are generated they are unstable and decarboxylate readily, providing much of the carbon dioxide produced in hydrocarbon oxidations. An in-depth article on aldehyde oxidation has been pubHshed (43). 

Reactor Configuration. The horizontal cross-sectional area of a reactor is a critical parameter with respect to oxygen mass-transfer effects in LPO since it influences the degree of interaction of the two types of zones. Reactions with high intrinsic rates, such as aldehyde oxidations, are largely mass-transfer rate-limited under common operating conditions. Such reactions can be conducted effectively in reactors with small horizontal cross sections. Slower reactions, however, may require larger horizontal cross sections for stable operation. 

D E L E P I N E Aldehyde Oxidation Mild oxidation of aldehydes to carboxylic acid using silver salts... 

Aldehyde oxidations occur through intermediate l/l-diols, or hydrates, which are formed by a reversible nucleophilic addition of water to the carbonyl group. Even though formed to only a small extent at equilibrium, the hydrate reacts like any typical primary or secondary alcohol and is oxidized to a carbonyl compound (Section 17.7). 

H2 EVOLUTION AND ALDEHYDE OXIDATION AT IB METALS IN ALKALINE SOLUTIONS... 

Mechanisms of aldehyde oxidation are not firmly established, but there seem to be at least two main types—a free-radical mechanism and an ionic one. In the free-radical process, the aldehydic hydrogen is abstracted to leave an acyl radical, which obtains OH from the oxidizing agent. In the ionic process, the first step is addition of a species OZ to the carbonyl bond to give 16 in alkaline solution and 17 in acid or neutral solution. The aldehydic hydrogen of 16 or 17 is then lost as a proton to a base, while Z leaves with its electron pair. 

Methylthiomethyl p-tolyl sulfone 257 was shown to react with various esters in the presence of excess NaH, affording compounds 263 which, upon reduction with NaBH and further treatment with alkali, can be converted to the corresponding aldehydes ". Oxidation of 263 with hydrogen peroxide gives S-methyl a-ketocarbothioates 264. ... 

Other mechanisms of ketone oxidation are also known and will be discussed in Chapter 8. Peracid, which is formed from aldehyde, oxidizes ketones with lactone formation (Bayer-Villiger reaction). 

Another peculiarity of the aldehyde oxidation is connected with the chemistry of the primary molecular product of oxidation, acyl peracid. The formed peracid interacts with the aldehydes to form peroxide with the structure RC(0)OOCH(OH)R [4,5]. This peroxide is unstable and decomposes into acids and free radicals. 

Ketones, like hydrocarbons and other organic compounds, are oxidized by dioxygen via the chain mechanism [4,62]. The carbonyl group weakens the adjacent C—H bond. Therefore, a peroxyl radical attacks the a-C—H bond as this bond is the most reactive in a ketone. The pecularities of ketone oxidation are the same as aldehyde oxidation. 

Aldehyde oxidation Aldehyde reduction Ketone reduction... 

Weiner H. Aldehyde oxidizing enzymes. In Jakoby WB, ed. Enzymatic Basis of Detoxication. New York, NY Academic Press 1980 261-280. 

Another reaction of dehalogenation, the oxidative dehalogenation of haloalkyl groups, summarized in Fig. 11.3,b (Chapt. 8 in [50]), has also been observed in mammals and other organisms. Here, the haloalkane is oxidized by a cytochrome P450 enzyme to form a hydroxylated intermediate that loses HX to become a carbonyl derivative. The latter is then reduced by dehydrogenases to the corresponding alcohol (Fig. 11.3,c), or, when the carbonyl derivative is an aldehyde, oxidation to the acid can occur (Fig. 11.3,c). 

The "silver mirror test" is used to distinguish an aldehyde from a ketone. Tollen s reagent, Ag(NH3)20H, acts as an oxidizing agent. When it is mixed with an aldehyde, the aldehyde oxidizes to the salt of a carboxylic acid. The silver ions in Tollen s reagent are reduced to silver atoms, and coat the glass of the reaction container with solid silver metal. 

Reductive coupling of 1,1-dimethylallene and 5-nitro-2-furancarboxaldehyde under a deuterium atmosphere provides the product of ferf-prenylation incorporating deuterium at the interior vinylic position (80% H). This result is consistent with a mechanism involving allene-aldehyde oxidative coupling. However, alternate pathways involving allene hydrometallation to furnish allyliridium species cannot be excluded on the basis of these data (Scheme 10). 

Scheme 13 Enantioselective carbonyl tert-prenylation from the alcohol or aldehyde oxidation level via iridium-catalyzed C-C bond-forming transfer hydrogenation...

More recently, using the cyclometallated iridium C,(7-benzoate derived from allyl acetate, 4-methoxy-3-nitrobenzoic acid and BIPHEP, catalytic carbonyl crotylation employing 1,3-butadiene from the aldehyde, or alcohol oxidation was achieved under transfer hydrogenation conditions [274]. Carbonyl addition occurs with roughly equal facility from the alcohol or aldehyde oxidation level. However, products are obtained as diastereomeric mixtures. Stereoselective variants of these processes are under development. It should be noted that under the conditions of ruthenium-catalyzed transfer hydrogenation, conjugated dienes, including butadiene, couple to alcohols or aldehydes to provide either products of carbonyl crotylation or p,y-enones (Scheme 16) [275, 276]. 

In an attempt to use an acyl anion equivalent to open an aziridine, Wu and co-workers isolated an unexpected ring opened product 316 (Eq. 31) [158], The authors found that the presence of oxygen was the determining factor between benzoin formation and ester formation. No desired ketones were ever formed. Various aromatic substituted aldehydes were treated under standard reaction conditions to afford esters in good yields. 4-Methoxybenzaldehyde provided product in only 40% yield, presumably due to the ease of aldehyde oxidation. 

Ellis and coworkers studied the effect of lead oxide on the thermal decomposition of ethyl nitrate vapor.P l They proposed that the surface provided by the presence of a small amount of PbO particles could retard the burning rate due to the quenching of radicals. However, the presence of a copper surface accelerates the thermal decomposition of ethyl nitrate, and the rate of the decomposition process is controlled by a reaction step involving the NO2 molecule. Hoare and coworkers studied the inhibitory effect of lead oxide on hydrocarbon oxidation in a vessel coated with a thin fQm of PbO.P l They suggested that the process of aldehyde oxidation by the PbO played an important role. A similar result was found in that lead oxide acts as a powerful inhibitor in suppressing cool flames and low-temperature ignitions.P l... 

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