1.1- Diols aldehyde hydrates

Ketones and aldehydes, because they are unsaturated, readily add water to give a 1,1-diol or hydrate ... 

Aldehyde oxidations occur through intermediate l/l-diols, or hydrates, which are formed by a reversible nucleophilic addition of water to the carbonyl group. Even though formed to only a small extent at equilibrium, the hydrate reacts like any typical primary or secondary alcohol and is oxidized to a carbonyl compound (Section 17.7). 

Aldehydes and ketones react with water to yield 1,1-diols, or geminal (gem) diols. The hydration reaction is reversible, and a gem diol can eliminate water to regenerate an aldehyde or ketone. 

Pyridyl aldehydes are reduced by a mechanism involving a preequilibrium in aqueous electrolytes.62,63 The rate-determining step for reduction was found to be the ionization of the gem-diol, or hydrated aldehyde. As a consequence, buffers were found to have an effect on the reduction of the 2-aldehyde (41 ).64... 

The Prins Reaction is the acid-catalyzed of addition aldehydes to alkenes, and gives different products depending on the reaction conditions. It can be thought of conceptually as the addition of the elements of the gem-diol carbonyl hydrate of the aldehyde across the double bond. 

Aldehyde Hydrates yem-DiolS Dissolving an aldehyde such as acetaldehyde in water causes the establishment of an equilibrium between the aldehyde and its hydrate. 

Aldehyde Hydrates ffem-Diols Dissolving an aldehyde such as acetaldehyde in water causes the establishment of an equilibrium between the aldehyde and its hydrate. This hydrate is in actuality a 1,1-diol, called a geminal diol (or simply a gm-diol). 

In addition to the physiological reaction, the reversible hydration of CO2 to bicarbonate (reaction 1, Scheme 1), CAs catalyze a variety of other reactions, such as the hydration of cyanate to carbamic acid, or of cyanamide to urea (reactions 2 and 3) the aldehyde hydration to gem-diols (reaction 4) the hydrolysis of carboxylic, or sulfonic (reactions 5, 6), as well as other less investigated hydrolytic processes, such as those described by equations 7-10 in Scheme 1 (Supuran et al., 1997 Supuran and Scozzafava, 2000b Guerri et al., 2000). It should be mentioned that the previously reported phosphatase activity of CA III was recently proved to be an artefact (EGm et al., 2000). It is unclear at this moment whether CA catalyzed reactions other than the CO2 hydration has physiological significance. 

Aldehydes and ketones react with water to form gem diols cahed hydrates. The equihbrium constant for hydrate formation depends on both steric and electronic factors. As in the case of cyanohydrins, the reaction with aldehydes is far more favorable than the reaction with ketones. 

The pinacol hydrate may be used (i) for conversion to pina-colone (see below), and (ii) to illustrate the oxidation of 1,2-diols to aldehydes or ketones by periodic acid (p. 145),... 

Terminal alkyne anions are popular reagents for the acyl anion synthons (RCHjCO"). If this nucleophile is added to aldehydes or ketones, the triple bond remains. This can be con verted to an alkynemercury(II) complex with mercuric salts and is hydrated with water or acids to form ketones (M.M.T. Khan, 1974). The more substituted carbon atom of the al-kynes is converted preferentially into a carbonyl group. Highly substituted a-hydroxyketones are available by this method (J.A. Katzenellenbogen, 1973). Acetylene itself can react with two molecules of an aldehyde or a ketone (V. jager, 1977). Hydration then leads to 1,4-dihydroxy-2-butanones. The 1,4-diols tend to condense to tetrahydrofuran derivatives in the presence of acids. 

Aldehyde or ketone Water Gemmal diol (hydrate) ... 

Geminal diol (Section 17 6) The hydrate R2C(OH)2 of an aldehyde or a ketone... 

The acid-catalvzed hydration reaction begins with protonation of the carbonyl oxygen atom, which places a positive charge on oxygen and makes the carbonyl group more electrophilic. Subsequent nucleophilic addition of water to the protonated aldehyde ot ketone then yields a protonated gem diol, which loses H+ to give the neutral product (Figure 19.5). 

Pyridinecarboxaldehyde, 3. Possible hydration of the aldehyde group makes the aqueous solution chemistry of 3 potentially more complex and interesting than the other compounds. Hydration is less extensive with 3 than 4-pyridinecarboxaldehyde but upon protonation, about 80% will exist as the hydrate (gem-diol). The calculated distribution of species as a function of pH is given in Figure 4 based on the equilibrium constants determined by Laviron (9). 

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. 

Reduction of the carboxylic acid group passes through the intermediate aldehyde. For a number of examples in the heterocyclic series, the aldehyde becomes a major product because it is trapped as the hydrated vfc.-diol form. Examples include imidazole-2-caiboxylic acid [139], thiazole-2-carboxylic acid [140] and pyridine-4-carboxylic acid [141] reduced in dilute aqueous acid solution. Reduction of imidazole-4-carboxylic acid proceeds to the primary alcohol stage, the aldehyde intermediate is not isolated. Addition of boric acid and sodium sulphite to the electrolyte may allow the aldehyde intermediate to be trapped as a non-reducible complex, Salicylaldehyde had been obtained on a pilot plant scale in this way by... 

Table 8.3 shows the solubilities of some potentially important aldehydes in the form of the Henry s law constant (H) and the effective Henry s law constant (// ) (Betterton and Hoffmann, 1988 Olson and Hoffmann, 1989). These aldehydes not only dissolve in aqueous solutions but also hydrate to form gera-diols (Buschmann et al., 1980, 1982) ... 

Problem 15.37 In acid, most aldehydes form nonisolable hydrates (gem-diols). Two exceptions are the stable chloral hydrate, Cl3CCH(OH)2, and ninhydrin. 

Aldehyde and ketone reacts with water in the presence of aqueous acid or base to form hydrate. A hydrate is a molecule with two hydroxyl groups on the same carbon. It is also called gem-diol. Hydration proceeds through the two classic nucleophilic addition mechanisms with water in acid... 

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