Benzaldehyde autoxidation

Generation of a radical through an oxidative process probably occurs in the initiation of the autoxidation of benzaldehyde (p. 319), which is catalysed by a number of heavy metal ions capable of one-electron transfers, e.g. Fe3 ... 

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 ... 

Recent detailed studies on autoxidation reactions have been published for tetralin 44-46) cumene and ethylbenzene 46,47) methyl oleate 48,49) and benzaldehyde 50h... 

Searching for other oscillatory autoxidation reactions led Druliner and Wasserman to use cyclohexanone as a substrate instead of benzalde-hyde (168). Unlike the simple stoichiometry found for the benzaldehyde reaction, the ketone gives at least six or more products, and the relative amounts of these vary substantially with the experimental conditions (Scheme 7). 

Thus the autoxidation of the aldehydes leads finally to acids. That a per-acid is first formed can be very easily shown in the case of acetaldehyde by the immediate liberation of iodine from potassium iodide solution which is caused hy this strong oxidising agent. In the case of benzaldehyde, which combines exceptionally rapidly with oxygen, it has been possible to trap the per-acid with acetic anhydride as benzoyl-acetyl peroxide (Nef) ... 

In salicylaldehyde the phenolic odour predominates. The aldehyde is much less liable to autoxidation than is benzaldehyde. 

The actual schemes of these reactions are very complicated the radicals involved may also react with the metal ions in the system, the hydroperoxide decomposition may also be catalysed by the metal complexes, which adds to the complexity of the autoxidation reactions. Some reactions, such as the cobalt catalysed oxidation of benzaldehyde have been found to be oscillating reactions under certain conditions [48],... 

Peracids form as transient species from the oxidation of benzaldehyde during autoxidation. For convenience we have chosen m-chloroperbenzoic acid (MCPBA) as our oxidant since this would be similar to the peracid formed from the very important intermediate 4-carboxybenzaldehyde formed during the oxidation of p-xylene (2). MCPBA would be formed in very low concentrations during oxidation hence we normally study the reaction of MCPBA with an excess of catalyst components i.e. MCPBA < pseudo first order conditions). The sequence of reactions that occurs when MCPBA is reacted with Co(II), Mn(II), and HBr has been previously discussed by Jones (9) in the presence of 5% water in acetic acid. We have repeated much of this work in 10% HjO/HOAc solutions and in general agree with his findings when one accounts for differences in temperatures, concentrations, and water concentrations. 

CyHeO, Mr 106.12, Z />101.3kPa 178.1 °C, d] 1.0415, Wp 1.5463, is the main, characteristic component of bitter almond oil. It occurs in many other essential oils and is a colorless liquid with a bitter almond odor. In the absence of inhibitors, benzaldehyde undergoes autoxidation to perbenzoic acid, which reacts with a second molecule of benzaldehyde to benzoic acid. 

Bawn and co-workers carried out detailed investigations of metal-catalyzed autoxidations of acetaldehyde313,314 and benzaldehyde.315 316a b The rate of chain initiation in the autoxidation of benzaldehyde catalyzed by cobalt acetate in acetic acid was equal to the rate of reaction of Co(III) with benzaldehyde in acetic acid in the absence of oxygen. Moreover, the onset of oxygen absorption coincided with the conversion of Co(II) to Co(III). The catalyst was maintained... 

Fig. 11.33. Regioselective Baeyer-Villiger rearrangement of an electron-poor aromatic aldehyde. This reaction is part of the autoxidation of benzaldehyde to benzoic acid. Both alternative reaction mechanisms are shown the [1,2]-rearrangement (top) and the /3-elimination (bottom).

According to Wittig and Lupin,160 the cyclic peroxide (151) is formed when a suspension of 1,4-dipotassium 1,1,4,4-tetraphenyl-butane in ether is treated with dry oxygen. The peroxide (152) is formed, though in small quantities, during the autoxidation of di-(p-anisyl)ethylene in benzaldehyde, by combination of two molecules of... 

Commercial benzaldehyde inhibited against autoxidation with 0.1% hydroquinone is usually satisfactory. If the material available is yellow or contains benzoic acid crystals it should be shaken with equal volumes of 5% sodium carbonate solution until carbon dioxide is no longer evolved and the upper layer dried over calcium chloride and distilled (bp 178-180°C), with avoidance of exposure of the hot liquid to air. The distillation step can be omitted if the benzaldehyde is colorless. 

Ground-state oxygen alone rarely oxidizes organic compounds. A classical example is the autoxidation of benzaldehyde to benzoic acid, a usually undesirable reaction that takes place even in the absence of light. Other examples of autoxidation without illumination are oxidations at the a positions with respect to aromatic rings or at tertiary carbons [47, 48, 49, 50] and the formation of alkyl hydroperoxides from alkyl dichloroboranes [57]. Some oxidations take place when a compound is treated with oxygen in the presence of bases [9, 52, 53]. 

Considering, for example, the autoxidation of benzaldehyde, the formation of one molecule of benzoic or of perbenzoic acid involves combination with one or two atoms of oxygen, respectively. In the autoxidation of an alkali sulfite, each molecule of sulfate formed utilizes one bound atom of oxygen. Thus,- from the analyses, the oxidation yield, RO, is obtained and is -reported as the ratio (multiplied by 100 in order to make the comparison easier) of the amount of reacted oxygen in excess to the amount of ozone consumed. 

In particular the autoxidation of benzaldehyde was investigated. Its choice as the initial subject for study was unfortunate, as the use of an impure sample of perbenzoic acid for determination of its absorption spectrum, not previously recorded, led to erroneous conclusions. These were later rectified after taking new measurements on a pure crystalline sample of the peracid (12). For the present study the two main bands at 1728 to 1730 and at 1270 cm.- are to be borne in mind. These bands made possible a demonstration of the acceleration of the autoxidation due to ozone and the influence of such acceleration in ozonide formation. Three spectral series (Figures 1, 2, and 3), obtained in collaboration with E. Dallwigk, are discussed below. 

In Figure 1 comparative spectra are shown for the autoxidation of benzaldehyde in the presence of either pure oxygen or oxygen with ozone in the indicated concentrations. 

In Figure 3 it is seen that, if a trans-stilhene solution containing benzaldehyde (spectrum I) is ozonized, the CO band due to the aldehyde at 1706 cm. i does not disappear but becomes much stronger (spectrum II). Therefore, a material with a CO band must have been produced which shows the same frequency as that of the aldehyde. Thus a protective action of the double bond against the autoxidation accelerated by ozone is plainly manifest. In the latest measurements it has been found that the stilbene double bond protects benzaldehyde against autoxidation due to oxygen alone thus substances with a double bond may act also as antioxidants. 

Furthermore, the protective action of the stilbene double bond becomes less and even ceases as soon as if the double bond were saturated by increasing ozonization. At this moment the two bands of perbenzoic acid appear and develop (spectra II and IV), indicating that autoxidation has set in. This proves that the CO band at 1706 cm. i must be ascribed to benzaldehyde, because perbenzoic acid, under these experimental conditions, can be derived only from the autoxidation of this aldehyde. 

Attempts were made to increase the ozone dilution further by using air-ozone mixtures of concentrations lower than Co3 = 0. 00001%. But difficulties were encountered because of atmospheric ozone at the altitude of Geneva (350 to 400 meters) there is about 0.000001% of ozone in the air. Thus, to prepare such mixtures of air and ozone, ordinary air was first deozonized by passing it through tubes heated to temperatures above 800° C. Furthermore an aldehyde reagent was used, butyralde-hyde dissolved in iso-octane, which is much more sensitive to autoxidation than benzaldehyde in carbon tetrachloride. 

In view of the susceptibility of benzaldehyde to autoxidation, it is remarkable that in the presence of DMSO the reaction stops at the aldehyde stage. In fact when a solution of benzaldehyde in DMSO was refluxed for 24 hrs. with air passing through the solution, only 1.6% of benzoic acid was isolated and 87% of benzaldehyde was recovered. Although air facilitates the oxidation, there is no oxygen uptake and isolation of dimethyl sulflde as such or as the mercuric chloride derivative (m.p. 150°) shows that DMSO is the oxidant. Several substituted benzyl alcohols were oxidized by the same method to the aromatic aldehydes. The one allylic alcohol... 

One is not observing only the autoxidation of toluene in these experiments but rather the co-oxidation of toluene with benzaldehyde, benzyl alcohol, benzyl acetate, and the benzyl bromide. Benzaldehyde, benzyl alcohol, and benzyl acetate are all more reactive than toluene [13]. 

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