Benzaldehyde hydrate formation

Assumption of a similar metabolic change might clear up some aspects of the biological oxidation of toluene to benzoic acid. Bray, Thorpe and White131 have studied the kinetics of the oxidation of both benzyl alcohol and benzaldehyde to benzoic acid. The velocity constant for the oxidation of the alcohol is 1.0, and that for the aldehyde is only 0.3, indicating that both cannot be intermediates in the oxidation of toluene. Since the alcohol has already been shown to be an intermediate, it follows that the aldehyde is not. They pointed out that hydrate formation and D-glu-curonic conjugation may precede oxidation. 

Benzaldehyde is especially prone to hydrate formation, and the first step of the Cannizzaro reaction is addition of hydroxide to benzaldehyde. Much of the time this reaction simply reverses to re-form hydroxide and benzaldehyde, but some of the... 

Oxidation of benzyl alcohol catalysed by chloroperoxidase exhibits a very high prochiral selectivity involving only the cleavage of the pro-S C-H bond. The reaction mechanism involved the transfer of a hydrogen atom to the ferryl oxygen of the iron-oxo complex. An a-hydroxy-carbon radical and the iron-hydroxy complex P-Fe -OH form. They may lead to the hydrated benzaldehyde or stepwise with the formation of the intermediate a-hydroxy cation. 

In 2008, Rostamizadeh and his coworkers reported that one-pot condensation of hydrazine hydrate with phenylisothiocyanate and benzaldehydes in the presence of [bmim][BF ] led to the formation of 1,3,4-thiadiazoles in excellent yields during relatively short reaction times (Fig. 12.3) [3]. A mechanism was proposed for these reactions (Fig. 12.4). From where it can be observed that after from formation of... 

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 ... 

In neutral aqueous solution at elevated temperatures, sodium dichromate oxidizes benzyl alcohol to benzaldehyde. The mechanism involves formation of a chromate ester and the unfavourable entropy of activation may be due to formation of a cyclic transition state (3) in which the carbonyl group is developed, while a substantial isotope effect (Ath/Atd = 9.4 0.9) indicates transfer of a proton. The oxidation of benzaldehyde to benzoic acid is substantially slower and involves a preequilibrium step in which the carbonyl is hydrated. A similar cyclic transition state is proposed, however. 

The oxidation of aromatic aldehydes could be complicated. In addition to direct oxidation to the dicarboxylic acid (phthalic acid), an aromatic aldehyde carbonyl could also be oxidized and then rearranged and hydrated to form a phenol via aryl formates (Dakin reaction)." Thus, when treating p-hydroxy-benzaldehyde, 25, with basic hydrogen peroxide, the following mechanism is actually involved (Fig. 10). 

The proposed mechanism assumes the formation of a cyclic complex between the reactive protonated PDC and lactic acid which undergoes C-C fission. The similarly retarded reactions of benzaldehyde and 4-nitrobenzaldehyde are second order in H+ ion. The decrease in rate with increasing dielectric constant of the solvent suggests a cation-dipole interaction. The proposed mechanism assumes the formation of a cyclic chromic ester between the hydrated benzaldehyde and the protonated PDC species, followed by C-H fission in the rate-determining step which is believed to feature a transition state with an electron-deficient reaction centre. " ... 

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