Equilibrium constants for hydration of carbonyl compounds

There have been numerous studies of the correlation between equilibrium acidity or basicity and the efficiency of various catalysts (see the discussion of the Br0nsted catalysis law in Chapter 4, section 4.6, p. 156). The correlations are generally good within related classes of acids and bases. Several water molecules may be involved in the transition state, in which case a concerted mechanism can be written for general acid-catalyzed hydration  

The occurrence of such processes cannot be detected directly from kinetic data, of course, since water is in large excess. 

Alcohols also undergo reversible addition reactions with aldehydes and ketones. The product of addition of one mole of alcohol to an aldehyde or ketone is referred to as a hemiacetal or hemiketal, respectively. Dehydration and addition of a second mole of alcohol gives an acetal or ketal. This second phase of the process can be catalyzed only by acids, since a necessary step is elimination of hydroxide ion from a tetrahedral intermediate. There is no low-energy mechanism for base assistance of this elimination step. For this reason, acetals and ketals are stable toward hydrolysis in alkaline aqueous solution. 

The equilibrium constants for addition of alcohols to carbonyl compounds to give hemiacetals or hemiketals show the same response to structural features as do those for hydration reactions. The magnitude of the equilibrium constants is slightly smaller than for hydration. For example, whereas K for addition of water to acetaldehyde is 1.06, K s for methanol and ethanol addition are 0.75 and 0.50M respectively. The overall equilibrium constants for formation of the acetals are 0.03 and 0.0125 M , respectively. 

The reverse reaction, hydrolysis of the ketal or acetal to the carbonyl compound and alcohol, has been studied in great detail. The mechanism is that specified in the preceding equations (in reverse). Among the evidence that supports this general mechanism are the following items  

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Table 7.4. Equilibrium Constants for Hydration of Carbonyl Compounds...

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