The Exchange of Carbonyl Compounds with Water

The Exchange of Carbonyl Compounds with Water A. Ketones and Aldehydes 

A kinetic study of this exchange must be done before a mechanism can be suggested, which should also include a study of the eifect of deuterium in the medium on the rate of exchange. 

In basic solution, simple addition of hydroxide ion to the carbonyl group seems to be the most probable mechanism for exchange. 

Enolization is unlikely as an essential step in the exchange since Senkus and Brown (1938) found that benzaldehyde (where enolization cannot occur) also undergoes O exchange with water. This has been confirmed by many later workers. The only recent studies on the exchange of 

Menon (1964) has measured the rate of acid- and base-catalysed oxygen exchange between p-substituted benzophenones and water in 80% dioxan-water. The rate data for the acid-catalysed oxygen exchange are summarized in Table 3. 

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By treating Nafion (NR-50), a perfluorinated acidic ion exchanger based on sulfonic acid groups, with scandium(III) chloride hexahydrate Kobayashi et al. generated a solid scandium-derived catalyst (29) (Nafion-Sc) that proved to be effective in al-lylation reactions of carbonyl compounds with tetraallyltin (Scheme 4.15). Since the catalyst is stable in both organic solvents and water, even unprotected carbohydrates could be transformed directly in aqueous solvents. The resulting homo-allylic alcohols were separated by simple filtration [97]. 

For some substances the half-wave potential of the oxidized form differs substantially from that of the reduced form. The shape of these waves, as well as the shifts of the half-wave potentials, are such as we would expect for a reversible process. Sometimes, the slope of the wave and the dependence of half-wave potentials on pH correspond to a transfer of a smaller number of electrons than is deduced from the limiting current. In such instances we assume that a part of the electrode process is mobile, but usually we describe such total processes as irreversible. Examples of such behaviour are the reductions of carbonyl compounds. For irreversible systems the exchange of water for an organic solvent, e.g. dioxane or dimethylformamide, can disclose whether or not, in the electrode process in aqueous solution, there is interaction with a molecule of water. 

Vardanyan [65,66] discovered the phenomenon of CL in the reaction of peroxyl radicals with the aminyl radical. In the process of liquid-phase oxidation, CL results from the disproportionation reactions of primary and secondary peroxyl radicals, giving rise to trip-let-excited carbonyl compounds (see Chapter 2). The addition of an inhibitor reduces the concentration of peroxyl radicals and, hence, the rate of R02 disproportionation and the intensity of CL. As the inhibitor is consumed in the oxidized hydrocarbon the initial level of CL is recovered. On the other hand, the addition of primary and secondary aromatic amines to chlorobenzene containing some amounts of alcohols, esters, ethers, or water enhances the CL by 1.5 to 7 times [66]. This effect is probably due to the reaction of peroxyl radicals with the aminyl radical, since the addition of phenol to the reaction mixture under these conditions must extinguish CL. Indeed, the fast exchange reaction... 

The rate of exchange of between a carbonyl compound and water should give information about the velocity of the reversible hydration process, as shown in the early work of Cohn and Urey (1938) with acetone. Herbert and Lauder (1938) showed that isotopic exchange is faster with acetaldehyde, but no quantitative information was obtained. It would... 

Systematics are also available for the 8 0-values of the compounds in queshon [56[ carboxyl and carbonyl functions in isotopic equilibrium with the surrounding water are, due to equilibrium isotope effects, enriched in 0 relative to this water by 19 and by 25 to 28%o, respectively. From here, the 8 0-values of natural alcohols, mostly descendants of carbonyl compounds, will have (maximally) similar 8 0-values, provided the precursors have attained isotopic equilibrium with water and their reduction has not been faster than their equilibration. Alcohols from addihon of water to C=C double bonds or from exchange of halogen functions by OH groups, typical for synthetic alcohols, will have 8 0-values close to or even below that of the water, due to kinetic isotope effects. The few available results [246, 289, 290] seem to confirm this expectation. The 8 0-values of natural (and also synthetic) esters and lactones can be, especially in the carbonyl group, extremely high (up to 50%o), probably as a consequence of an intramolecular kinetic isotope effect on the activation of the carboxyl function. 

A number of reactions of carbonyl compounds require the presence of a base, but the base is not used up in the reaction and is therefore acting as a catalyst. The aldol condensation considered earlier (reaction 4.20) is an example, and another is the base-catalysed exchange of lsO between water and esters (reaction 4.29) which accompanies hydrolysis. The base reacts with the carbonyl compound in the first stage of the reaction, either... 

The COj species in the HT interlayer could be exchanged with OH ions by calcination at 723 K and hydration at room temperature. A spinel phase of Mg-Al mixed oxide obtained after the calcination transforms into the original layered structure during the hydration. This reconstruction is known as the memory effect of HT materials. The reconstructed HT catalyzed the Knoevenagel condensation of various aldehydes with nitriles in the presence of water [119]. The reconstracted HT also showed an aqueous Michael reaction of nitriles with a,p-unsaturated compounds. The layered double-hydroxide-supported diisopropylamine catalyzed the Knoevenagel condensation of aromatic carbonyl compounds with malononitrile or ethyl cyanoacetate [120]. This solid base could be recycled at least four times, and exhibited activity for aldol, Henry, Michael, transesterification, and epoxidation of alkenes. 

The most commonly used protected derivatives of aldehydes and ketones are 1,3-dioxolanes and 1,3-oxathiolanes. They are obtained from the carbonyl compounds and 1,2-ethanediol or 2-mercaptoethanol, respectively, in aprotic solvents and in the presence of catalysts, e.g. BF, (L.F. Fieser, 1954 G.E. Wilson, Jr., 1968), and water scavengers, e.g. orthoesters (P. Doyle. 1965). Acid-catalyzed exchange dioxolanation with dioxolanes of low boiling ketones, e.g. acetone, which are distilled during the reaction, can also be applied (H. J. Dauben, Jr., 1954). Selective monoketalization of diketones is often used with good success (C. Mercier, 1973). Even from diketones with two keto groups of very similar reactivity monoketals may be obtained by repeated acid-catalyzed equilibration (W.S. Johnson, 1962 A.G. Hortmann, 1969). Most aldehydes are easily converted into acetals. The ketalization of ketones is more difficult for sterical reasons and often requires long reaction times at elevated temperatures. a, -Unsaturated ketones react more slowly than saturated ketones. 2-Mercaptoethanol is more reactive than 1,2-ethanediol (J. Romo, 1951 C. Djerassi, 1952 G.E. Wilson, Jr., 1968). 

The occurrence of isotopic exchange of between water and carbonyl compounds has been observed to take place slowly with acetone (Cohn and Urey, 1938) and much more rapidly with acetaldehyde (Herbert and Lauder, 1938). This gives qualitative evidence for reversible hydration (since no other reasonable mechanism exists for isotopic exchange), but gives no quantitative information about the equilibrium position. Similarly, the fact that exchange occurs in the unhydrolysed ester during the hydrolysis of carboxylic esters (Bender, 1951) shows that the species RC(0H)20R is a stable intermediate rather than a transition state. 

These observations emphasize the fact that gem-diols are usually unstable and decompose to carbonyl compounds. However, it can be demonstrated that hydrate formation does occur by exchange labelling of simple aldehyde or ketone substrates with 0-labelled water. Thus, after equilibrating acetone with labelled water, isotopic oxygen can be detected in the ketone s carbonyl group. 

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