Aldehyde autoxidation

Complexes such as [Ni(16)] are known to stoichiometrically interact with 02 to give 1 1 adducts and subsequently the autoxidized Ni111 species (compare Section 6.3.4.10.2(v)). 5 Such systems have been tested for the NiIII-catalyzed cleavage of DNA (see Section 6.3.4.10.2(v)). It has been suggested that Ni11 macrocycle complexes with rather low Nin/Nim reduction potentials can be active inhibitors of aldehyde autoxidation.156... 

Therefore, aldehyde autoxidation produces an efficient autoinitiator. 

Clinton NA, Kenley RA, Traylor TG. Aldehyde autoxidative carbon dioxide evolution. J Am Chem Soc 1975 97 3752-3757. 

Metals can play a clear but small role in such epoxidations or esterifications with a sacrificial aldehyde. First, it has long been known that the free radical aldehyde autoxidation can be initiated and therefore accelerated by metal ions. For instance, high concentrations of peracid can be obtained with dissolved Fe catalysts (5). As a rule, the presence of a metal allows autoxidation at considerably lower temperature than for the noninitiated reaction. The exact nature of the metal is not critical. Second, epoxidations by a peracid may be metal centered however, at the typical temperatures of the Mukuyiama epoxidation, the uncatalyzed reaction of peracid and olefin is usually fast enough to make the metal redundant for this second step. 

Thus it was established by the infrared study of aldehyde autoxidation that the first product obtained is the peracid. It does not follow, however, that other intermediates are not formed—for example, radicals in a chain reaction—but their instability is such that they cannot be detected by infrared spectrography. 

The initiation step was proposed to proceed via the formation of the oxygenated adduct, like Co -0-0, which reacts with aldehyde in the rate-determining step [26]. However, the fact that the induction time is more for CoPc as compared to C0W12 (the latter is unable to form any adducts), shows that species different from oxygenated cobalt adducts may promote the chain initiation. We believe that for most cobalt catalysts studied these species are Co(III) forms of the catalyst produced by one-electron oxidation of Co(II) initial forms with peroxy acid, which in turn is produced in the course of aldehyde autoxidation. Usually, the end of the induction period coincides with the change of the reaction mixture colour expected for Co(III) appearance. 

Work on metal ion catalyzed aldehyde autoxidation done up to the mid-1970 s [5-19] has been reviewed by Sheldon and Kochi [1]. 

Organic peroxy acids are well-known reagents for aromatic hydroxylation of arenes [42, 43]. m-Chloroperbenzoic acid was applied to prepare quinones in low to moderate yields [54]. Peroxy acids can be generated in situ from and carboxylic acids or via aldehyde autoxidation [42]. Arnold et al. reported the synthesis of menadione in ca. 30% yield by using 30% in acetic acid [55]. Following this strategy, Orita et al. oxidized a range of arenes by an excess of in formic... 

Table III. Termination rate constants for aldehyde autoxidation ...

Hydroxyalkyl peroxyesters also have been isolated from the autoxidation products of aldehydes and by esterification of hydroxyhydroperoxides (44). 

Another method for producing petoxycatboxyhc acids is by autoxidation of aldehydes (168). The reaction is a free-radical chain process, initiated by organic peroxides, uv irradiation, o2one, and various metal salts. It is terrninated by free-radical inhibitors (181,183). In certain cases, the petoxycatboxyhc acid forms an adduct with the aldehyde from which the petoxycatboxyhc acid can be hberated by heating or by acid hydrolysis. If the petoxycatboxyhc acid remains in contact with excess aldehyde, a redox disproportionation reaction occurs that forms a catboxyhc acid ... 

The peroxycarboxyhc acid can be generated m situ by autoxidation of aldehydes, either in the presence of anhydrides or an acyl chloride and a base, eg, sodium carbonate, or basic ion-exchange resins (44,187,188,210) ... 

Common impurities found in aldehydes are the corresponding alcohols, aldols and water from selfcondensation, and the corresponding acids formed by autoxidation. Acids can be removed by shaking with aqueous 10% sodium bicarbonate solution. The organic liquid is then washed with water. It is dried with anhydrous sodium sulfate or magnesium sulfate and then fractionally distilled. Water soluble aldehydes must be dissolved in a suitable solvent such as diethyl ether before being washed in this way. Further purification can be effected via the bisulfite derivative (see pp. 57 and 59) or the Schiff base formed with aniline or benzidine. Solid aldehydes can be dissolved in diethyl ether and purified as above. Alternatively, they can be steam distilled, then sublimed and crystallised from toluene or petroleum ether. 

Functional groups that stabilize radicals would be expected to increase susceptibility to autoxidation. This is illustrated by two cases that have been relatively well studied. Aldehydes, in which abstraction of the aldehyde hydrogen is fecile, are easily autoxidized. The autoxidation initially forms a peroxycarboxylic acid, but usually the corresponding carboxylic acid is isolated because the peroxy acid oxidizes additional aldehyde in a... 

All of the reactions discussed up till now involve the autoxidation of methylbenzenes to the corresponding carboxylic acids. From a practical viewpoint it would also be interesting to devise a process for the production of the corresponding aldehyde. Unfortunately, as noted earlier, the oxidizability of ArCHO is about four orders of magnitude higher than ArCH3 which essentially precludes the selective production of the aldehyde when O2 is the oxidant. With all other oxidants, on the other hand, the rate of oxidation of ArCHO is lower than that of ArCH3 (ref. 24) (Fig. 21). 

Aldehydes, and particularly aromatic ones, are highly susceptible to autoxidation thus benzaldehyde (97) is rapidly converted into benzoic acid (98) in air at room temperature. This reaction is catalysed by light and the usual radical initiators, but is also highly susceptible to the presence of traces of metal ions that can act as one-electron oxidising agents (cf. p. 306), e.g. Fe3 , Co3 , etc ... 

The autoxidation of aldehydes, and of other organic compounds, may be lessened considerably by very careful purification—removal of existing peroxides, trace metal ions, etc.—but much more readily and effectively by the addition of suitable radical inhibitors, referred to in this context as anti-oxidants. The best of these are phenols and aromatic amines which have a readily abstractable H atom, the resultant radical is of relatively low reactivity, being able to act as a good chain terminator (by reaction with another radical) but only as a poor initiator (by reaction with a new substrate molecule). 

Several cases of spontaneous ignition after exposure to air of fine coke particles removed from filter strainers on a petroleum refinery furfural extraction unit have been noted. This has been associated with the use of sodium hydrogen carbonate (bicarbonate) injected into the plant for pH control, which produced a pH of 10.5 locally. This would tend to resinify the aldehyde, but there is also the possibility of a Cannizzaro reaction causing conversion of the aldehyde to furfuryl alcohol and furoic acid. The latter, together with other acidic products of autoxidation of the aldehyde, would tend to resinily the furfuryl alcohol. Pyrolysis GLC showed the presence of a significant proportion of furfuryl alcohol-derived resins in the coke. The latter is now discarded into drums of water, immediately after discharge from the strainers, to prevent further incidents. 

A violent explosion occurred at the start of bulb-tube distillation of a 2 g sample of the crude aldehyde (b.p. 9774 mbar) at 75°C/0.13 mbar [1], It seems likely that the crude product could have formed peroxide during the 2 day s storage (in a closed but not hermetically sealed vessel) between preparation and distillation. The structure is such that the proton on C-2, which effectively has 2 benzyl substituents and an adjacent carbonyl function, seems rather susceptible to autoxidation [2], See other aldehydes, peroxidisable compounds... 

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