Oxidation of benzaldehyde

Since Liebig [1], in 1835, observed that benzaldehyde was transformed into benzoic acid when left in the presence of air, a great deal of research has been done on the oxidation of this aldehyde. Kinetic investigations have been made on two types of system, one consisting of the oxidation of benzaldehyde alone and the other of its co-oxidation with other reactants. 

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Kinetic isotope effect. The oxidation of benzaldehyde by permanganate ions is believed to occur by hydride abstraction. What value of k /kD do you predict for C6H5CHO/ C6H5CDO For C6H5CHO/C6D5CHO ... 

The chromic acid oxidation of benzaldehyde to benzoic acid in aqueous solution was examined by Graham and Westheimer, and in acetic acid solution by Wiberg and Mill . With water as solvent the kinetics are... 

Although the fate of Cr(IV) is uncertain, (cf. the alcohol oxidation), some characteristics of the intermediate chromium species have been obtained by Wiberg and Richardson from a study of competitions between benzaldehyde and each of several substituted benzaldehydes. The competition between the two aldehydes for Cr(VI) is measured simply by their separate reactivities that for the Cr(V) or Cr(IV) is obtained from estimation of residual aldehyde by a C-labelling technique. If Cr(V) is involved then p values for oxidation by Cr(VI) and Cr(V) are 0.77 and 0.45, respectively. An isotope effect of 4.1 for oxidation of benzaldehyde by Cr(V) was obtained likewise. 

Wiberg and Lepse have examined the oxidation of benzaldehyde by chromyl... 

Two studies of the Ce(IV) perchlorate oxidation of benzaldehyde in aqueous acetic acid have been reported . The rate law is of the form... 

Recently Mosher and Driscoll 2 have noted that the polymerization of acrylonitrile can be observed during the chromic acid oxidation of 2,2-dimethyl-l-phenyl-l-propanol. The polymerization is caused by radicals formed during the oxidation of benzaldehyde (which is one of the cleavage product of phenyl-1-butylcarbinol). The oxidation of benzaldehyde is due to the chromium(IV), most probably, or chromiun(V) intermediates. 

The oxidation of benzaldehyde with chromic acid was investigated by Graham and Westheimer using a spectrophotometric method, and the following rate equation was found... 

To decide whether the reaction involves 1- or 2-electron transfers, i.e. chromium-(rv) or chromium(V) is formed first, the induced oxidation of manganese(II) was investigated. When sodium perchlorate was used to maintain a constant ionic strength, the rate of oxidation of benzaldehyde dropped to one-half of the original rate in the presence of manganese(II) ions. On the contrary, when magnesium perchlorate was used as the neutral salt, the rate was reduced to of its original value. This peculiar observation, however, has not been interpreted. 

Knowledge of stoichiometry of the induced reaction could help to distinguish whether chromium(V) or chromium(IV) species are involved in the oxidation of benzaldehyde. Thus, the Cr(V) hypothesis predicts that for each molecule of benzaldehyde oxidized two molecules of manganese dioxide should be formed, whereas the Cr(IV) predicts that one molecule of manganese dioxide should be formed for each two molecules of benzaldehyde oxidized. Unfortunately, the attempt to determine the stoichiometry of the induced reaction failed because the oxidized manganese species was not precipitated during the reaction presumably due to formation of acetate complexes in the concentrated acetic acid solution. 

Zellner G, H Kneifel, J Winter (1990) Oxidation of benzaldehydes to benzoic acid derivatives by three Desul-fovibrio strains. Appl Environ Microbiol 56 2228-2233. 

The phenomenon of chemical induction was intensively studied by Jorissen [33-37]. He discovered that indigo was not oxidized by dioxygen but was simultaneously oxidized in the presence of oxidized triethylphosphine or benzaldehyde. He measured the factor of chemical induction in these reactions as equal to unity. Later, he proved that the oxidation product of benzaldehyde, benzoic peracid, did not oxidize indigo under conditions of experiment. This shows that a very active intermediate was formed during the oxidation of benzaldehyde and that it was not perbenzoic acid. Engler assumed peroxide to be in two forms, namely, an active moloxide A02 and a more stable peroxide. A new correct interpretation of chemical induction in oxidation reactions was provided later by the chain theory of oxidation of organic compounds (see later). 

Photochemically induced oxidation of benzaldehyde was proved to be the chain reaction (4> >> 1) H. Backstrom [51]... 

The study of benzaldehyde and cyclohaxanone co-oxidation showed the formation of s-caprolactone as the main product of cyclohexanone oxidation [5]. Cyclohexanone was found not to react practically with peroxyl radicals under mild conditions. The oxidation of benzaldehyde produces perbenzoic acid. The latter oxidizes the benzaldehyde to benzoic acid and cyclohexanone to s-caprolactone. 

AM Novak. Liquid-Phase Oxidation of Benzaldehyds and Tetrahydrobenzaldehydes and Cooxidation of Benzaldehydes with Organic Compounds. Thesis Dissertation, Institute Physical Organic Chemistry, Donetsk, 1979 [in Russian]. 

The oxidation of benzaldehyde with dioxygen in the presence of Co(II) and bromide ion also shows non-linear kinetic phenomena. The net reaction of the oxidation process is given as follows ... 

FAD dependent metalloenzyme, oxidation of benzaldehyde to benzoic acid... 

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],... 

Examples have been described of cyclization of bis(azostilbene) derivatives by reaction with hydrogen sulfide and hydrochloric acid, by irradiation, and by heat. 2,4,5-Triphenyltriazole is formed in each case. Nonoxidative cyclization of bis arylhydrazones is also reported. A reaction related to these is the oxidation of benzaldehyde... 

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. 

A number of years ago it was observed that the perbenzoic acid oxidation of benzaldehyde phenylhydrazone afforded a high-melting product of low solubility which was termed benzaldehyde phenylhydrazone oxide. More recent work has shown that this, as well as related oxidation products of other phenylhydrazones, is, in fact, the cis isomer of the corresponding azoxy compounds [4, 7]. 

The most direct evidence that negative catalysis sometimes works in this way in ordinary thermal reactions, and, therefore, incidentally that the chain mechanism can operate in such reactions, has been found by Backstrom. In the photochemical oxidation of benzaldehyde, heptalde-hyde, and of solutions of sodium sulphite, there are very large numbers of molecules transformed for each quantum of light absorbed, amounting respectively to 10,000,15,000, and 50,000 for the three reactions. Such deviations from Einstein s law show that the light probably sets up chain reactions. These photochemical changes are markedly subject to the action of inhibitors, which presumably cut short the chains. Backstrom establishes the important... 

It has been reported that Mo-P oxides show a good performance in oxidation of butenes to maleic anhydride [26]. On the other hand, Bordes et al. [27] have reported that U-Mo oxides with Mo-rich compositions are effective as catalysts for oxidation of butenes to maleic anhydride. These findings suggest that the functions required for oxidation of toluene are similar to those required for oxidation of butenes to maleic anhydride. However, the V-P oxides are not effective for toluene oxidation. Possibly, the consecutive oxidation of benzaldehyde cannot be suppressed with V Og-containing catalysts. Even over the Mo-P and U-Mo oxides, benzaldehyde is degraded, to a certain extent. 

The main by-product with Mo-P-based oxides is maleic anhydride [20,21], while that with U-Mo oxides is carbon oxides. These by-products are formed by the consecutive oxidation of benzaldehyde. Possibly, maleic anhydride formed initially is decomposed promptly to carbon oxides over U-Mo oxides, while Mo-P-based oxides are inactive for the decomposition of maleic anhydride. 

It may be concluded that oxidation of toluene to benzaldehyde is more difficult than oxydative dehydrogenation of olefins, because toluene is relatively stable and, further more, benzaldehyde is very unstable. Relatively good performances are obtained with Mo-P and U-Mo oxides, because the consecutive oxidation of benzaldehyde is suppressed satisfactorily with these oxides. 

Exercise 16-35 A radical-chain reaction similar to that described for the air oxidation of benzaldehyde occurs in the peroxide-initiated addition of aldehydes to alkenes (see Table 10-3). Write a mechanism for the peroxide-induced addition of ethanal to propene to give 2-pentanone. 

The reaction mechanism for A-oxidation by performic acid has been studied by AMI calculation methods.174 The iminium salt A-mcthyl-3,4-dihydroisoquinolinium p-toluenesulfonate has been used to catalyse the oxidation of the azo dye calmagite by peracetic acid. The mechanism at pH 10 involves peracid oxidation of the quinolinium ion to form an oxaziridinium salt, which then acts as an oxygen transfer agent for oxidation of cahnagite.175 The presence of lithium salts affects the course of the reaction determining the formation of benzoyl peroxide and benzoic acid as final products in the oxidation of benzaldehyde by perbenzoic acid.176,177... 

Several studies have tackled the structure of the diketopiperazine 1 in the solid state by spectroscopic and computational methods [38, 41, 42]. De Vries et al. studied the conformation of the diketopiperazine 1 by NMR in a mixture of benzene and mandelonitrile, thus mimicking reaction conditions [43]. North et al. observed that the diketopiperazine 1 catalyzes the air oxidation of benzaldehyde to benzoic acid in the presence of light [44]. In the latter study oxidation catalysis was interpreted to arise via a His-aldehyde aminol intermediate, common to both hydrocyanation and oxidation catalysis. It seems that the preferred conformation of 1 in the solid state resembles that of 1 in homogeneous solution, i.e. the phenyl substituent of Phe is folded over the diketopiperazine ring (H, Scheme 6.4). Several transition state models have been proposed. To date, it seems that the proposal by Hua et al. [45], modified by North [2a] (J, Scheme 6.4) best combines all the experimentally determined features. In this model, catalysis is effected by a diketopiperazine dimer and depends on the proton-relay properties of histidine (imidazole). R -OH represents the alcohol functionality of either a product cyanohydrin molecule or other hydroxylic components/additives. The close proximity of both R1-OH and the substrate aldehyde R2-CHO accounts for the stereochemical induction exerted by RfOH, and thus effects the asymmetric autocatalysis mentioned earlier. 

The oxidation of benzaldehyde is also catalyzed by nickel(II) acetate317a>b. Reaction (237) is faster than reaction (236). Hence, oxidation of nickel(II) is... 

Figure 5.35 shows the retarded oxidation of benzaldehyde in the presence of antioxidants at 30°C. It is interesting to see a common intercept in the linear plots. 

Although alkaline conditions are normally employed, for reasons of safety it is occasionally advantageous to operate under neutral conditions where, for example, oxidation of benzaldehydes to benzoic acids may be desired rather than the Dakin reaction.210 Conversely alkyl benzaldehydes, which under most conditions would be expected to yield the corresponding benzoic acids, have been reported to undergo the Dakin-type reaction yielding alkyl phenols in the presence of strong acids.220... 

A number of other peroxygen reagents have been used for aldehyde oxidation. In particular, solid adducts of hydrogen peroxide such as urea-hydrogen peroxide (UHP) can generate peracids conveniently in situ.230 This can be applied to the oxidation of benzaldehydes (Figure 3.47). The products are those expected to form typical peracid reactions and are obtained under mild conditions in good yield. 

Benzaldehyde dehydrogenase (BDH), a hypothetical enzyme, catalyzes the oxidation of benzaldehyde to benzoic acid. The enzyme has a flavin prosthetic group that accepts electrons from the substrate upon oxidation ... 

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