Benzaldehyde oxidation inhibition

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. 

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

Copper-iron-polyphthalocyanine [251,252] showed a specific catalysis for the oxidations of saturated aldehydes and substituted benzaldehydes with oxygen. The catalytic reaction was solvent dependent so that tetrahydrofuran, ethanol, acetonitrile, ethyl acetate and anisole inhibited benzaldehyde oxidation while oxidation occurred readily in benzene or acetone. Benzaldehyde was catalytically oxidized with copper-iron-polyphthalocyanine and oxygen to give a quantitative yield of a mixture of perbenzoic (61%) and benzoic (39%) acids. Reaction was carried out at 30 °C and atmospheric pressure of oxygen and exhibited no induction period. By contrast p-methyl and p-chlorobenzaldehyde had induction periods of 8 and 15 min respectively while no oxidation of p-substituted benzaldehydes was observed when the para-substituent was NO2, OH, OCH3, or N(CH3)2. 

Benzaldehyde is easily oxidised by atmospheric oxygon giving, ultimately, benzoic acid. This auto-oxidation is considerably influenced by catalysts tiiose are considered to react with the unstable peroxide complexes which are the initial products of the oxidation. Catalysts which inhibit or retard auto-oxidation are termed anti-oxidants, and those that accelerate auto-oxidation are called pro-oxidants. Anti-oxidants find important applications in preserving many organic compounds, e.g., acrolein. For benzaldehyde, hydroquinone or catechol (considerably loss than U-1 per cent, is sufficient) are excellent anti-oxidants. 

Nevertheless, for the production of the flavour-active aromatic alcohol derivatives, such as the corresponding aldehydes and acids, metabolic engineering approaches have to compete with conventional oxidative biocatalysis starting from the natural alcohol as a substrate. For instance, the whole-cell oxidation system based on Pichia pastor is AOX already described in Sect. 23.4.1.2 can also be used to convert benzyl alcohol to benzaldehyde in aqueous media although product inhibition restricted the final product concentration to about 5 g L h indicating the need for aqueous-organic two-phase reaction media [51]. Phenylacetalde-... 

Tsuruya and co-workers (83,84) recently reported that addition of alkaline earth metals (e.g., Ca, Sr, and Ba) to an Ag/SiOi catalyst by a coimpregnation method enhanced the catalytic activity of the partial oxidation of benzyl alcohols into benzaldehydes, with production of only small amounts of byproducts (carbon dioxide, toluene, and benzene). The formation of carbonaceous material was thought to be inhibited by the alkaline earth metals, which also helps to disperse the metallic silver and facilitate oxygen adsorption. This effect causes the formation of an oxygenated silver surface that is generally believed to be responsible for the partial oxidation of benzyl alcohol. 

Derivative benzoates and parabenzoates have been used primarily in fruit juices, chocolate syrup, pie fillings, pickled vegetables, relishes, horseradish, and cheese (Barbosa-Canovas et al., 2003). Other foodstuffs where sodium benzoate is used include soft drinks, baked goods, and lollipops (Poulter, 2007). Benzaldehyde and benzoic alcohol are better known to be yeast inhibitors. Benzoic acid has been found to release fewer protons than sulphite, nitrite, or acetic acid and it may be speculated that benzoic acid is not a classic weak-acid preservative. However, due to a lower pKa value, benzoic acid releases three to four times more protons than sorbic acid. This is a sizable concentration of protons although not as much as other weak-acid preservatives (Stratford and Anslow, 1998). Inhibition of growth is strongly pH-dependent and most effective under acidic conditions. Under these conditions the protonated form of the acid is predominantly found (Visti, Viljakainen, and Laakso, 2003). Another unexpected discovery was that benzoic acid appears to be a pro-oxidant. This was unexpected as it is a well-known fact 2-hydroxybenzoic acid (or salicylic acid) acts as a scavenger of free radicals in vivo (Piper, 1999). 

Chemical evidence for the importance of reaction (229) was first obtained by Waters and Wickenham-Jones [153,154] for reactions in oxidizing benzaldehyde inhibited by 2,6-dimethylphenol. They found that the phenol was converted to the corresponding diphenoquinone... 

Hermans provided a number of insights into the reaction mechanism (Scheme 15.4). One key observation is that benzyl nitrite, formed via the reaction between HNO2 and benzyl alcohol, appears and decays over the reaction time course. Under anaerobic conditions with catalytic amberlyst-15 but without NO, benzyl nitrite fully converts to benzaldehyde and HNO, implicating its role as an intermediate (Scheme 15.4, steps I and II). In the anaerobic reaction, N2O was detected as a by-product. N2O can form through dimerization and then dehydration of HNO. Because N2O is inert and cannot be converted back into active NO species, it is important to inhibit its formation. The presence of NO2 gas was shown to reduce N2O formation, leading to the proposal that NO2 is an intermediate for the oxidation of HNO back to HNO2 (Scheme 15.4, step III). 

Charge-transfer spectra of 2,4,6-triarylthiopyrylium cations in the presence of stable carbanions, for example [C(CN)3], have been measured. Irradiation of the thiopyrylium perchlorate (43 X = CIO4) in methanol, in the presence of oxygen, leads to a mixture of products, including thiophenol, benzaldehyde, benzoic acid, and methyl benzoate. Methylene blue inhibits the oxidation. ... 

The oxidation of benzaldehyde in the presence of cobaltous acetate has been studied in detail by Marta, Boga and co-workers [226-230]. A radical chain mechanism was involved and inhibition of this reaction both by jS-naphthol and by cobaltous ion at high concentration, have been observed. The initiation step was found to involve the decomposition of a Co CPhCOaH) complex to give Co species which were the reactive intermediates. The rate constant and heat of formation of the Co (PhC03H) complex were determined. Bawn and Jolley [225] have shown that at low Co concentration, the oxidation of benzaldehyde follows rate law, equation (177). 

The kinetics of the oxidation of benzaldehyde in the presence of [Pd(PPh3)2(02)] was studied [265]. The initial rate of oxygen consumption was described by equation (180), where Ci and are constants. The reaction was inhibited by free phosphine and markedly retarded by perbenzoic acid while added benzoic acid had no effect. 

Inhibitors of chain reactions. A simple reaction cannot be retarded by minor additives of various substances. The chain reaction is retarded by an inhibitor, viz., the substance reacting with active centers and thus terminating chains. Inhibition of chain reactions was predicted by Christiansen and used for the first time by H. Backstrom as evidence for the chain mechanism of oxidation of sulfite and benzaldehyde (1926). 

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