Ketone oxidation mechanism

Akubuiro and Wagner [85,86] proposed ketone oxidation mechanisms at the surface of an activated carbon. Ketones are oxidized to produce peroxides, which are very unstable and decompose with strong exothermic reactions. These by-products give carboxylic acids, aldehydes and/or diketones. An example of the reaction pathways is shown in Fig. 12. 

Degradation of polyolefins such as polyethylene, polypropylene, polybutylene, and polybutadiene promoted by metals and other oxidants occurs via an oxidation and a photo-oxidative mechanism, the two being difficult to separate in environmental degradation. The general mechanism common to all these reactions is that shown in equation 9. The reactant radical may be produced by any suitable mechanism from the interaction of air or oxygen with polyolefins (42) to form peroxides, which are subsequentiy decomposed by ultraviolet radiation. These reaction intermediates abstract more hydrogen atoms from the polymer backbone, which is ultimately converted into a polymer with ketone functionahties and degraded by the Norrish mechanisms (eq. 

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). 

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. 

Synthesis of 63 and 64 supports the olefin oxidation mechanisms in Fig. 16. These mechanisms have several important and noteworthy points about Ptm chemistry (1) olefins coordinate to Ptm at the axial position, which is contrasted to the -coordination of olefins perpendicular to the square-planar coordination plane of Ptn. Olefin coordination to Pt(III) should also be contrasted to the fact that olefins do not coordinate to Pt(IV). (2) Platinum111 is strongly electron-withdrawing, and the coordinated olefins receive nucleophilic attack. (3) The alkyl ce-carbon on the Ptm undergoes nucleophilic attack in aqueous solution, whereas in aprotic solvent, aldhyde (and possibly also ketone in other cases) is produced by reductive elimination. 

Figure 7.10 (top) shows the FT-IR spectrum of freshly prepared C60D36. The exposure to air after 1 day causes alterations in the spectrum (Fig. 7.10, middle). In particular it can be noticed the reduction of the intensity of the C-D stretching band at 2,092 cm-1 and the complete disappearance of the C-D bending at 966 cm-1. Evidences of oxidation can be inferred by the C=0 stretching band at 1,710 cm-1 and by the C-OH and C-OOH bending at about 1,040 cm-1 supporting the allylic oxidation mechanism. After 3 days exposure to air an increase in the relative intensity of the ketone, hydroxyl and hydroperoxide bands can be observed (Fig. 7.10, bottom). 

Dess-Martin A5-iodane 44 is an extremely useful reagent for the conversion of primary and secondary alcohols to aldehydes and ketones at 25 °C [70]. It does not oxidize aldehydes to carboxylic acids under these conditions. It selectively oxidizes alcohols in the presence of furans, sulfides, and vinyl ethers. The oxidation mechanism involves a facile ligand exchange with alcohols, followed by reductive /1-elimination. 

This was not always the case for the ketones which can be produced by non-oxidative mechanisms, and are also more volatile than their corresponding aldehydes. 

Dess-Martin- Qxio6xmnQ (DMP, 73) is an oxidizing agent that oxidizes the alcohol at C-7 to the corresponding ketone. The mechanism of this transformation is similar to that of the IBX oxidation shown earlier. 

Once formed, alcohols esterify to some extent with the acids generated in an oxidation reaction. Except for lactones, esters do not appear to be generated directly in oxidation mechanisms [10, 37, 38]. The Bayer-Villiger reaction of intermediate peracids and ketones is sometimes proposed as a source of esters [39] but it appears to be too slow to be a significant source except in the case of cycloparaffins [10, 40]. The ester group and its immediate neighboring groups appear to be remarkably resistant to oxidation. 

A great deal of research has been directed towards the elucidation of this oxidation mechanism which is summarized in Figure 2. The free fatty acids liberated by the Penicillium roqueforti lipase are oxidized to -keto acids which undergo decarboxylation to form the methyl ketones. Under certain circumstances, the methyl ketones are reduced to the corresponding secondary alcohols (11). 

The mechanism of the oxidation may be interpreted by the following steps. (1) Oxygen attacks the methyl group at the aid of the longest open chain of the hydrocarbon to form water and an aldehyde, probably through the decomposition of initially formed peroxides. (2) The aldehyde is oxidized to a lower aldehyde, water, carbon monoxide, or carbon dioxide. (3) In the case of the branched isomers, this process continues until a branch in the molecule is reached, giving rise to a ketone instead of an aldehyde as the product. (4) Oxidation at low temperature slows down at this stage since ketones oxidize with more difficulty than aldehydes. 

Thermal oxidative degradation of PE and PE nanocomposites has been extensively studied over the past decades [26-30], It has been reported that the main oxidation products of PE are aldehydes, ketons, carboxylic acids, esters and lactones [26, 27], According to Lacoste and Carlsson [28], P-scission plays an important role in thermal oxidation of UHMWPE. Notably, the feasibility of intra-molecular hydrogen abstraction by the peroxy radicals for polyethylene has been questioned in frames of a thermal oxidation mechanism proposed by Gugumus [29, 30],... 

Fig. 12. Probable oxidation mechanisms of methyl ethyl ketone (reprinted with permission from [85,86], copyright 1992,1993 American Chemical Society).

Feii)4(B-PW9O34)2] 0- also catalyzes the oxidation of cyclohexane, methylcyclohexane and hexane with formation of the corresponding alcohols and ketones. The mechanism of hydrocarbon oxidation in the tetranuclear iron sandwich polyoxotungstate - hydrogen peroxide svstem will be discussed. 

Selenium dioxide has been used quite widely to effect oxidation of ketones and aldehydes to a-dicarbonyl compounds. The reaction often gives high )delds of products when there is a single type of CH2 group adjacent to the carbonyl group. In unsymmetriceil ketones, oxidation usually occurs at the CH2 that is most readily enolized. The mechanism of the reaction is thought to involve formation of a... 

Also, with iron porphyrin deposited at the cathode, alkanes have been catalytically oxidized to ketones and alcohols in an electrochemical cell in the presence of oxygen An oxidation mechanism similar to that of a P-450 oxidation is assumed in this case. 

---