Benzaldehydes oxidation

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. 

Salts of transition metals are widely used in technological processes for the preparation of various oxygen-containing compounds from hydrocarbon raw materials. The principal mechanism of acceleration of RH oxidation by dioxygen in the presence of salts of heavy metals was discovered by Bawn [46 19] for benzaldehyde oxidation (see Chapter 1). Benzaldehyde was oxidized with dioxygen in a solution of acetic acid, with cobalt acetate as the catalyst. The oxidation rate was found to be [50] ... 

It was in 1924 when Christiansen [3] put forward the conception of the chain mechanism of oxidation and explained the action of antioxidants via chain termination by the antioxidant [3]. Three years later, Backstrom and coworkers [4—6] experimentally proved the chain mechanism of benzaldehyde oxidation (see Chapter 1) and the mechanism of antioxidant (hydroquinone) action via chain termination. The systematic study of the oxidation kinetics of esters of nonsaturated acids was performed by Bolland and ten Have [7,8], They observed in the kinetic experiments that substrates are oxidized by the chain mechanism with chain propagation via the cycle of reactions (see Chapter 2). 

The assignment of the excited state of benzophenone as a triplet which could act as a sensitizer was made by Hammond and Moore in 1959 (equation 49)/ and this led to a great surge in radical study using photochemical techniques. The role of photoexcited benzophenone as a diradical initiator for benzaldehyde oxidation was previously shown explicitly by Backstrom in 1934 (equation 34). ... 

BENZALDEHYDE. [CAS 100-52-7], C6H5CHO, formula weight 106.12, colorless liquid, mp bp -26°C, bp 179°C, sp gr 1.046. Sometimes referred to as artificial almond oil or oil of bitter almonds, hen/aldehyde has a characteristic nullike odor. The compound is slightly soluble in H20, but is miscible in all proportions with alcohol or ether. On standing in air, benzaldehyde oxidizes readily to benzoic acid. Commercially, benzalde-hyde may be produced by (1) heating benzal chloride C H CHCL with calcium hydroxide. (2i heating calcium benzoate and calcium formate ... 

Fig. 9. Effect of solution pH on the redox behavior of an Ru02 aqueous solution interface. (— -), From work with an Ru02/Ti02 film [209]. The upper lines are for the Ru(IV)/Ru(VI) transition as obtained from voltammograms for pure RuOz (on Ti) in a range of borate (pH > 8) and phthalate (pH < 8) buffer solutions O and refer to the cathodic and anodic peak maxima respectively. (- -), Variation of the half-wave potential for benzaldehyde oxidation (O.lmoldm-3, scan rate = 1.5mVs 1) on pure Ru02 in buffered 10% f-butanol in water mixtures. The voltammograms outlined on the left and right for Ru02 in acid and base, respectively [207],...

Benzaldehyde oxide, syntheeie. 111 e-Benzalfluorene oxJds, oondanastiDB with methanol, 296 Banzol p-pwiboxyadetopbencme,... 

Above a temperature of about 450°C the catalyst deactivates due to the formation of heavier products which remain on the zeolite, explaining the lowering of the selectivity above this temperature, but at lower temperatures benzaldehyde selectively transforms to benzoic acid. Phenol and benzene are the main by-products of benzaldehyde oxidation with selectivities up to about 10% each. The other by-products observed in toluene oxidation, including phenylbenzoate, were also detected in benzaldehyde oxidation. 

Progress in understanding the role of antioxidants has paralleled the understanding of oxidation kinetics the first real insight into antioxidant mechanisms occurred roughly at the time that Backstrom [2] defined the radical chain character of benzaldehyde oxidation. 

These differences in reactivity are, depending on the workers concerned, usually attributed to polar effects. Nevertheless, it seems apparent that interpretation is made uncertain by the extreme sensitivity of benzaldehyde oxidation to fortuitous inhibition phenomena. In other words, it is not certain that these rather surprising results cannot be attributed to the quantitative treatment of data from experiments involving considerable difficulties. 

This mechanism was unequally established for the case of benzaldehyde oxidation at room temperature in solutions of glacial acetic acid [9]. The kinetic equation obtained for the reaction is... 

However, later it became clear that in most cases the assumption of the stable peroxide cannot by itself explain the phenomena observed. For example, though benzaldehyde oxidation initiates the coupled oxidation of indigo, specially synthesized peroxybenzoic acid can hardly oxidize indigo, and if so, the reaction proceeds far slower than it does in the process of coupled oxidation. 

Figure 5. Benzaldehyde oxidation over sample c. benzaldehyde conversion i residue CO 2 space-time 0.4 mgcat. min/ml.

Benzoic acid has long been considered to be formed by benzaldehyde oxidation in the presence of oxygen, but was not bebeved to form under anaerobic conditions [95]. Here, when the reaction was performed under anaerobic conditions, benzoic acid was not detected in the effluent by gas chromatography (GC) analysis, but in situ ATR-IR spectra clearly showed its presence as an adsorbed benzoate species, indicated by bands at 1600, 1546,1422, and 1393 cm [94]. The fact that benzoate species are detected on the catalyst surface, but not in the effluent, suggests that these species are very strongly bound possibly at basic sites on the AljOj support The formation of benzoic acid/benzoate under anaerobic conditions was speculated to result from hydration of benzaldehyde via a germinal diol, followed by a dehydrogenation. This pathway is summarized as follows ... 

A non-radical mechanism for benzaldehyde oxidation catalyzed by Pd(PPh )2(02 been proposed [3] but was later refuted in favor of... 

Propanol inhibits benzaldehyde oxidation catalyzed by Fe(acac), ... 

Interest in benzaldehyde oxidation has recently been revived by the observation of oscillations [23,24] and propagating reaction fronts [25-27] due to its autocatalytic nature. 

S. Meenakshisundaram et al, Oxidation of alcohols using supported gold and gold-palladium nanoparticles, Faraday Discuss., 2010,145(0), 341-356. M. Sankar et al.. The benzaldehyde oxidation paradox explained hy the interception of perojy radical by benzyl alcohol, Nat. Commun., 2014, 5, 3332-3337. 

Table 6.2. Computed target functional (/) and selectivity (5) for benzaldehyde oxidation into peroxybenzoic acid depending on the and 4 magnitudes...

The JUC-Z4 composite with heptamolybdate soft-oxometalate has been used as a catalyst for benzaldehyde oxidation. This composite has also been... 

---