Oxidation of aldehyde

Aldehydes are readily oxidized to carboxylic acids by a number of reagents, including those based on Cr(VI) in aqueous media. 

Mechanistically, these reactions probably proceed through the hydrate of the aldehyde and follow a course similar to that of alcohol oxidation. 

Hydrates of aldehydes are more easily oxidized than alcohols, which is why special reagents such as PCC and PDC (Section 15.9) have been developed for oxidizing primary alcohols to aldehydes and no further. PCC and PDC are effective not only because they are sources of Cr(VI), but also because they are used in nonaqueous media (dichloromethane). By keeping water out of the reaction mixture, the aldehyde is not converted to its hydrate, which is the necessary intermediate that leads to the carboxylic acid. 

Alcohol oxidation, especially of ethanol, is one of the most common of all biological processes. Two key enzymes, both classified as dehydrogenases, are involved. The first catalyzes the oxidation of ethanol to acetaldehyde, the second catalyzes the oxidation of acetaldehyde to acetic acid. 

Acetaldehyde is toxic and responsible for many of the adverse effects attributed to ethanol. Too much ethanol produces acetaldehyde faster than it can be oxidized to acetic acid and leads to elevated acetaldehyde levels. 

Aldehydes give a positive Tollens test that is, they react with Ag to form RCOOH and Ag. When the reaction is carried out in a glass flask, a silver mirror is formed on its walls. Other functional groups give a negative Tollens test, because no silver mirror forms. 

Aldehydes are oxidized selectively in the presence of other functional groups using silverOD oxide in aqueous ammonium hydroxide (Ag20 in NH4OH). This is called Tollens reagent. 

Oxidation with Tollens reagent provides a distinct color change, because the Ag reagent is reduced to silver metal (Ag), which precipitates out of solution. 

What product is formed when each compound is treated with either AggO, NH4OH or Na2Cr207, H2SO4, H2O (a) C6H5CH2OH (b) CH3CH(0H)CH2CH2CH2CH0  

Aldehydes are easily oxidized to yield carboxylic acids, but ketones are generally inert toward oxidation. The difference is a consequence of structure aldehydes have a -CHO hydrogen that can he abstracted during oxidation, but ketones do not. 

Many oxidizing agents, including KMn04 and hot HNO3, convert aldehydes into carboxylic acids, but Cr03 in aqueous acid is a more common choice. The oxidation takes place rapidly at room temperature. 

1 Oxidation of Aldehydes Oxidation of Glyoxal to Glyoxylic Acid 

Glyoxylic acid (CHOCOOH), used in the preparation of fine chemicals (e. g., vanillin and penicillin), is prepared industrially by oxidation of glyoxal with nitric acid. An attempt was made to replace this stoichiometric process by oxidation of glyoxal with air on platinum catalysts [57-59]. In a first series of experiments, catalysts containing different platinum metals (Pt, Ir, Pd, Rh, Ru) prepared on the same active carbon and with the same particle size (1-2 nm) were compared. The initial rate of reaction increased in the sequence 0 = Ru Rh Pd Ir Pt, which is similar to that of the redox potentials of these elements. 

Oxidation of aqueous solutions (10% wlw ) of 3-hydroxypropionaldehyde (HPA) in the presence of 3 % Pd/C catalysts at pH 8 produces malonic acid. Complete conversion, with 96.7 % yield, was achieved with large amounts of catalyst (33% wlw Pd relative to HPA) [91]. Starting from 3-hydroxypropionic acid the malonic acid selectivity was 95.4 % at 97 % conversion. In the presence of 5 % Pt/C catalysts (28 % wlw Pt with respect to HPA), aqueous solutions (10% wlw) of 3-hydroxypropionaldehyde were oxidized into 3-hydroxypropionic acid, an intermediate used to prepare pharmaceutical and agricultural products. The best yield, obtained without pH regulation, was 92.9% at 97.2% conversion [92]. 

A solution of methacrolein in methanol and aqueous soda solutions were continuously oxidized by O2 at 5 bar and 80 °C in a reactor packed with Pd-Bi/ Si02-Mg0-Al203 catalyst. Methyl methacrylate was obtained with 90.8 % selectivity at 63.4 % conversion [93]. 

Although dilute sodium hydroxide is often employed to maintain the alkaline 

Application of Hydrogen Peroxide for the Synthesis of Fine Chemicals 

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 

More complex pre-formed pereacids, such as 3-chloroperbenzoic acid223 or the magnesium salt of monoperoxyphthalic acid,224 may also be used to oxidize aromatic aldehydes, either to carboxylic acids or phenols. 

Peroxymonosulfuric acid (Caro s acid, H2S05) and its salts may be used to oxidize aldehydes. Although early results were poor compared with the use of organic peracids, good yields of esters have been obtained when reactions are carried out in the presence of alcohols.225 Unsaturated and aromatic aldehydes undergo analogous reactions. It is believed that hemiacetal formation occurs in these reactions, and that it is this species which is oxidized, rather than the aldehyde. 

TaUe 13.10 Catalytic oxidation of aminoalcohols with carbon dispersed gold in the presence of alkali. [Substrate]=0.4 M, substrate/metal = 1000, substrate/NaOH=l, p02=3 bar, r=70°C, t=2h. 

From the above experiments, we concluded that gold catalysis is quite sensitive to the nature of the substrate and further research is needed to optimising the 

The oxidation of liquid aldehydes can also be carried out in the absence of solvent. In the case of 2-methylpropanal and n-heptanal we observed that a smooth reaction occurs, with TOF values of 4000-7500 h , using air instead of pure O2, at low temperature (25-70°C), thus allowing safer reaction conditions. 

Aromatic aldehydes behave differently from the aliphatic ones and, in particular, we noted a not yet understood structure effect related to the ring substituents. In fact, comparing different substrates, a strong deactivating effect was observed for the hydroxy group in the ortho and para positions, as shown in Table 13.12. 

In Chapter 15, we saw that primary alcohols are oxidized to aldehydes, which are then easily oxidized to acids. Under the same conditions, secondary alcohols are oxidized to ketones, but no further. This difference in reactivity distinguishes primary from secondary alcohols. 

Tollens reagent is a basic solution of a silver ammonia complex ion. When an aldehyde is added to a test tube containing Tollens reagent, the aldehyde is oxidized and deposits metallic silver as a mirror on the wall of the test tube. 

Adding Tollens reagent to each of the following isomeric carbonyl compounds gives a clear result that distinguishes between the two isomers. 

Benedict s solution contains cupric ion (Cu ) as a complex ion in a basic solution like Tollens reagent, it converts aldehydes to carboxylic acids. In this reaction, Cu is reduced to Cu, which forms as a brick-red precipitate, CU2O. Benedicts solution has the characteristic blue color of Cu, which fades as the red precipitate of CU2O forms. Benedicts solution is basic, and in a basic solution, a carboxyhc acid is converted to its conjugate base, that is, a carboxylate anion. 

Fehling s solution, which contains Cu as a different complex ion in a basic solution, also oxidizes aldehydes but not ketones. Either of the reagents can be used to distinguish between compounds such as the following isomeric aldehyde and ketone. 

A Tollens test is usually done on a small scale, but it can also create a silver mirror on a large object. 

Unlike ketones, aldehydes are easily oxidized to carboxylic adds by common oxidants such as chromic add, chromium trioxide, permanganate, and peroxy adds. Aldehydes oxidize so easily that air must be excluded from their containers to avoid slow oxidation by atmospheric oxygen. Because aldehydes oxidize so easily, mild reagents such as Ag20 can oxidize them selectively in the presence of other oxidiz-able functional groups. 

Silver ion, Ag , oxidizes aldehydes selectively in a convenient functional-group test for aldehydes. The Tollens test involves adding a solution of silver-ammonia complex (the Tollens reagent) to the unknown compound. If an aldehyde is present, its oxidation reduces silver ion to metallic silver in the form of a black suspension or a silver mirror deposited on the inside of the container. Simple hydrocarbons, ethers, ketones, and even alcohols do not react with the Tollens reagent. 

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By oxidation of aldehydes with potassium permanganate solution, for example ... 

Oxidation of aldehydes (Section 17 15) Aldehydes are particularly sensitive to oxidation and are converted to carboxylic acids by a number of oxidizing agents in eluding potassium permanganate and chromic acid... 

The lower temperatures and reduced degree of oxygen starvation in LPO (vs VPO) generally reduce carbon monoxide production markedly by promoting reaction 18 and suppressing reaction 21. As a consequence, acids, from further oxidation of aldehydes, are usually the main products. 

Sofid sodium permanganate monohydrate has been shown to be a selective synthetic reagent (156). It is typically used in hexane for the heterogeneous oxidation of aldehydes, alcohols, and sulfides. Synthetic methodology based on crystal surfaces exhibited greater selectivity, higher yield, and easier work-up as compared to aqueous permanganate reactions. 

Although alcohol dehydrogenases (ADH) also catalyze the oxidation of aldehydes to the corresponding acids, the rate of this reaction is significantly lower. The systems that combine ADH and aldehyde dehydrogenases (EC 1.2.1.5) (AldDH) are much more efficient. For example, HLAD catalyzes the enantioselective oxidation of a number of racemic 1,2-diols to L-a-hydroxy aldehydes which are further converted to L-a-hydroxy acids by AldDH (166). 

D E L E P I N E Aldehyde Oxidation Mild oxidation of aldehydes to carboxylic acid using silver salts... 

Oxidation of aldehyde or ketones to t, 2-dlcarbonyi compounds wSh Se02 (sometimes oxidation to o nsaturated ketones). 

The NAD- and NADP-dependent dehydrogenases catalyze at least six different types of reactions simple hydride transfer, deamination of an amino acid to form an a-keto acid, oxidation of /3-hydroxy acids followed by decarboxylation of the /3-keto acid intermediate, oxidation of aldehydes, reduction of isolated double bonds, and the oxidation of carbon-nitrogen bonds (as with dihydrofolate reductase). 

At the present time, the greatest importance of covalent hydration in biology seems to lie in the direction of understanding the action of enzymes. In this connection, the enzyme known as xanthine oxidase has been extensively investigated.This enzyme catalyzes the oxidation of aldehydes to acids, purines to hydroxypurines, and pteridines to hydroxypteridines. The only structural feature which these three substituents have in common is a secondary alcoholic group present in the covalently hydrated forms. Therefore it was logical to conceive of this group as the point of attack by the enzyme. 

N.A. Anastasijevic, H. Baltruschat, and J. Heitbaum, On the hydrogen evolution during the electrochemical oxidation of aldehydes at lb metals, Electrochim. Acta 38(8), 1067-1072 (1993). 

Dichromate oxidation of secondary alcohols produces ketones in good yield, with little additional oxidation. For example, CH,CH2CH(OH)CH3 can be oxidized to CH CH2COCH3. The difference between the ease of oxidation of aldehydes and that of ketones is used to distinguish them. Aldehydes can reduce silver ions to form a silver mirror—a coating of silver on test-tube walls—with Tollens reagent, a solution of Ag1" ions in aqueous ammonia (Fig. 19.3) ... 

The oxidation of aldehydes to carboxylic acids can proceed by a nucleophilic mechanism, but more often it does not. The reaction is considered in Chapter 14 (14-6). Basic cleavage of (3-keto esters and the haloform reaction could be considered at this point, but they are also electrophilic substitutions and are treated in Chapter 12 (12-41 and 12-42). 

For a review of metal ion-catalyzed oxidative cleavage of alcohols, see Trahanovsky, W.S. Methods Free-Radical Chem. 1973, 4, 133. For a review of the oxidation of aldehydes and ketones, see Verter, H.S. in Zabicky The Chemistry of the Carbonyl Group, pt. 2 Wiley NY, 1970, p. 71. 

Hassall, C. W., The Baeyer-Villiger oxidation of aldehydes and ketones. Organic Reactions. 9, 73-106, 1957. 

Oxidation of aldehydes and organic acids by chromium VI) a. Benzaldehyde... 

Oxidation of Aldehydes to Carboxylic Acids - Addition of Oxygen... 

Drivers for Performing Oxidation of Aldehydes to Carboxylic Adds in Micro Reactors... 

Oxidation of Aldehydes to Carboxylic Acids Investigated in Micro Reactors Cas/liquid reaction 26 [CL 26) Homc eneously catalyzed oxidation of butyraldehyde to butyric acid... 

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