Addition of Water to Form Hydrates

This section and Sections 17-7 and 17-8 introduce the reactions of aldehydes and ketones with water and alcohols. These compounds attack the carbonyl group through the mechanisms just outlined, utilizing either acid or base catalysis. 

Water is capable of attacking the carbonyl function of aldehydes and ketones. The transformation can be catalyzed by either acid or base and leads to equilibration with the corresponding geminal diols, RC(OH)2R, also called carbonyl hydrates. 

In the base-catalyzed mechanism, the hydroxide functions as the nucleophile. Water then protonates the intermediate adduct, a hydroxy alkoxide, to give the product diol and to regenerate the catalyst. 

In the acid-catalyzed mechanism, the sequence of events is reversed. Here, initial protonation facilitates nucleophihc attack by the weak nucleophile water. Subsequently, the catalytic proton is lost to reenter the catalytic cycle. 

As these equations indicate, hydrations of aldehydes and ketones are reversible. The equilibrium lies to the left for ketones and to the right for formaldehyde and aldehydes bearing inductively electron-withdrawing substituents. For ordinary aldehydes, the equilibrium constant approaches unity. 

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Aldehydes and ketones undergo acid-catalyzed addition of water to form hydrates. Electron donation and bulky substituents decrease the percentage of hydrate present at equilibrium. Most hydrates are too unstable to be isolated. Acid-catalyzed addition of alcohol to aldehydes forms hemiacetals and acetals, and to ketones forms hemiketals... 

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