Hydrates of carbonyl

Buschmann, H.-J., H.-H. Fuldner, and W. Knoche, The Reversible Hydration of Carbonyl Compounds in Aqueous Solution. Part I. The Keto/Gem-Diol Equilibrium, Ber. Bunsenges. Phys. Chem., 85, 41- 44 (1980). 

Carbonic anhydrase is a zinc(II) metalloenzyme which catalyzes the hydration and dehydration of carbon dioxide, C02+H20 H+ + HC03. 25 As a result there has been considerable interest in the metal ion-promoted hydration of carbonyl substrates as potential model systems for the enzyme. For example, Pocker and Meany519 studied the reversible hydration of 2- and 4-pyridinecarbaldehyde by carbonic anhydrase, zinc(II), cobalt(II), H20 and OH. The catalytic efficiency of bovine carbonic anhydrase is ca. 108 times greater than that of water for hydration of both 2- and 4-pyridinecarbaldehydes. Zinc(II) and cobalt(II) are ca. 107 times more effective than water for the hydration of 2-pyridinecarbaldehyde, but are much less effective with 4-pyridinecarbaldehyde. Presumably in the case of 2-pyridinecarbaldehyde complexes of type (166) are formed in solution. Polarization of the carbonyl group by the metal ion assists nucleophilic attack by water or hydroxide ion. Further studies of this reaction have been made,520,521 but the mechanistic details of the catalysis are unclear. Metal-bound nucleophiles (M—OH or M—OH2) could, for example, be involved in the catalysis. 

The hydration of carbonyl compounds, along with hemiketal and ketal or hemicacetal and acetal formation, were studied in K. B. Wiberg, K. M. Morgan and H. Maltz, J. Am. Chem. Soc., 116, 11067 (1994). 

Acid-catalyzed hydration of carbonyl Preliminary examination... 

Fig. 12 Mechanisms for base-catalyzed, uncatalyzed, and acid-catalyzed hydration of carbonyl compounds.

Table 8 AG1 for hydration of carbonyl compounds by acid-catalyzed, base-catalyzed, and uncatalyzed mechanisms ...

As already mentioned, S-hydroxyketones may be present in the open-chain form or as an isomeric cyclic hemiketal, depending on their substitution pattern. For example, the polyhydroxylated S-hydroxyketone, which D-fructose can be considered to be, is present exclusively in the form of hemiketals (Figure 7.13). Responsible for this equilibrium position is the fact that the carbonyl group involved contains an electron-withdrawing group in both a positions (cf. our discussion of the substituent effects on the equilibrium position of the hydration of carbonyl compounds Section 7.2.1). 

Another interesting possibility for the second step involves the reversal of a carbonyl addition. Examples are the dehydration of a gem-dihydroxy compound (hydrate of carbonyl compound)... 

Hemihydrate A species formed by the condensation of two hydrates of carbonyl compounds to give a compound of general formula RCH(0H)-0-CHR(0H). 

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