Oxygen of aldehydes

The carbonyl oxygen of aldehydes and ketones can form hydrogen bonds with the pro tons of OH groups This makes them more soluble m water than alkenes but less solu ble than alcohols... 

The Wittig reaction consists in the replacement of carbonyl oxygen of aldehydes and ketones by a methylene group with the aid of phosphine-methylenes resulting in the formation of cis or trans olefines. The reaction proceeds through the nucleophilic addition of Wittig reagent (phosphine methylene) across the > C = O bond and formation of an intermediate cyclic. 

The carbonyl oxygen of aldehydes and ketones is less basic than that of an alcohol by several powers of 10. We have just seen above that this arises because the lone pair electrons of the carbonyl oxygen are in orbitals that are approximately sp in character, and are more tightly held than the alcohol lone pairs in sp orbitals. The neutral carbonyl group is thus... 

The a-oxygenation of aldehydes is a highly versatile reaction that affords the oxygenated products in high yield and high enantioselectivity. In 2(X)3, three different groups (Zhong [331], MacMillan [332], and Hayashi [333]) independently reported the use of nitrosobenzene for this reaction. The reaction is also applicable to ketones,... 

The first electrophilic source of oxygen introduced for the proline-catalyzed a-oxygenation of aldehydes and ketones was nitrosobenzene, based on the use of this reagent in the asymmetric metal-catalyzed oxidation of tin enolates [12]. A number of research groups, including those of Zhong [12a], MacMillan [13b],... 

Figure 5.5 Mechanism of the proline-catalysed a-oxygenation of aldehydes and ketones...

Lewis acids react with the carbonyl oxygen of aldehydes or ketones to form the corresponding ate complex, which is also an oxocarbenium ion. When the generic carbonyl compound 1 reacts with BF3, charge-transfer complex 3 is the oxocarbenium ion product (see Chapter 16, Section 16.3). This ate complex is also stabilized by resonance, as shown in 3. 

In contrast to oxidation in water, it has been found that 1-alkenes are directly oxidized with molecular oxygen in anhydrous, aprotic solvents, when a catalyst system of PdCl2(MeCN)2 and CuCl is used together with HMPA. In the absence of HMPA, no reaction takes place(100]. In the oxidation of 1-decene, the Oj uptake correlates with the amount of 2-decanone formed, and up to 0.5 mol of O2 is consumed for the production of 1 mol of the ketone. This result shows that both O atoms of molecular oxygen are incorporated into the product, and a bimetallic Pd(II) hydroperoxide coupled with a Cu salt is involved in oxidation of this type, and that the well known redox catalysis of PdXi and CuX is not always operalive[10 ]. The oxidation under anhydrous conditions is unique in terms of the regioselective formation of aldehyde 59 from X-allyl-A -methylbenzamide (58), whereas the use of aqueous DME results in the predominant formation of the methyl ketone 60. Similar results are obtained with allylic acetates and allylic carbonates[102]. The complete reversal of the regioselectivity in PdCli-catalyzed oxidation of alkenes is remarkable. 

The oxidation of terminal alkenes with an EWG in alcohols or ethylene glycol affords acetals of aldehydes chemoselectively. Acrylonitrile is converted into l,3-dioxolan-2-ylacetonitrile (69) in ethylene glycol and to 3,3-dimetho.xy-propionitrile (70) in methanol[28j. 3,3-Dimethoxypropionitrile (70) is produced commercially in MeOH from acrylonitrile by use of methyl nitrite (71) as a unique leoxidant of Pd(0). Methyl nitrite (71) is regenerated by the oxidation of NO with oxygen in MeOH. Methyl nitrite is a gas, which can be separated easily from water formed in the oxidation[3]. 

The most obvious way to reduce an aldehyde or a ketone to an alcohol is by hydro genation of the carbon-oxygen double bond Like the hydrogenation of alkenes the reac tion IS exothermic but exceedingly slow m the absence of a catalyst Finely divided metals such as platinum palladium nickel and ruthenium are effective catalysts for the hydrogenation of aldehydes and ketones Aldehydes yield primary alcohols... 

Lone pair donation from the hydroxyl oxygen makes the carbonyl group less elec trophilic than that of an aldehyde or ketone The graphic that opened this chapter is an electrostatic potential map of formic acid that shows the most electron rich site to be the oxygen of the carbonyl group and the most electron poor one to be as expected the OH hydrogen... 

The carbon-nitrogen triple bond of nitriles is much less reactive toward nucleophilic addition than is the carbon-oxygen double bond of aldehydes and ketones Strongly basic nucleophiles such as Gngnard reagents however do react with nitriles in a reaction that IS of synthetic value... 

Acetaldehyde is a highly reactive compound exhibiting the general reactivity of aldehydes (qv). Acetaldehyde undergoes numerous condensation, addition, and polymerisation reactions under suitable conditions, the oxygen or any of the hydrogens can be replaced. 

The lower temperatures and reduced degree of oxygen starvation in LPO (vs VPO) generally reduce carbon monoxide production markedly by promoting reaction 18 and suppressing reaction 21. As a consequence, acids, from further oxidation of aldehydes, are usually the main products. 

Bacterial concentrations have also been determined by using the enzyme-catalyzed chemiluminescent reaction of reduced flavin mononucleotide (FMN) with oxygen and aldehydes. The detection limit was reported to be 10 ceUs of E. coli, which contains 7 x 10 g of FMN per ceU (303). 

Noncatalytic oxidation of propylene to propylene oxide is also possible. Use of a small amount of aldehyde in the gas-phase oxidation of propylene at 200—350°C and up to 6900 kPa (1000 psi) results in about 44% selectivity to propylene oxide. About 10% conversion of propylene results (214—215). Photochemical oxidation of propylene with oxygen to propylene oxide has been demonstrated in the presence of a-diketone sensitizers and an aprotic solvent (216). 

Reduction of Aldehydes and Ketones to Hydrocarbons. Deep hydrogenation of aldehydes and ketones removes the oxygen functionahty and produces the parent hydrocarbons. 

The three T s of combustion—time, temperature, and turbulence—govern the speed and completeness of the combustion reaction. For complete combustion, the oxygen must come into intimate contact with the combustible molecule at sufficient temperature and for a sufficient length of time for the reaction to be completed. Incomplete reactions may result in the generation of aldehydes, organic acids, carbon, and carbon monoxide. 

Like aldehydes, ketone functions take precedence over alcohol functions, double bonds, halogens, and alkyl groups in determining the parent name and direction of numbering. Aldehydes outrank ketones, however, and a compound that contains both an aldehyde and a ketone carbonyl group is nfflned as an aldehyde. In such cases, the carbonyl oxygen of the ketone is considered an oxo-substituent on the main chain. 

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