Catalysis of hemiacetal and hydrate formation

Acid and base catalysis of hemiacetal and hydrate formation... 

ACID AND BASE CATALYSIS OF HEMIACETAL AND HYDRATE FORMATION... 

Carbonyl addition reactions include hydration, reduction and oxidation, the al-dol reaction, formation of hemiacetals and acetals (ketals), cyanohydrins, imines (Schiff bases), and enamines [54]. In all these reactions, some activation of the carbonyl bond is required, despite the polar nature of the C=0 bond. A general feature in hydration and acetal formation in solution is that the reactions have a minimum rate for intermediate values of the pH, and that they are subject to general acid and general base catalysis [121-123]. There has been some discussion on how this should be interpreted mechanistically, but quantum chemical calculations have demonstrated the bifunctional catalytic activity of a chain of water molecules (also including other molecules) in formaldehyde hydration [124-128]. In this picture the idealised situation of the gas phase addition of a single water molecule to protonated formaldehyde (first step of Fig. 5) represents the extreme low pH behaviour. 

Since rapid conversion of hemiacetal to acetal requires more acidic conditions than does formation of the hemiacetal, it is possible to measure the rate of hemiacetal production without complication from the second stage of the reaction.52 As might be expected, the hemiacetal formation displays characteristics similar to those of hydration general acid and general base catalysis are observed.53... 

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