12.11 : アルデヒドおよびケトンと水: 水和物の形成

Aldehydes and ketones undergo hydration reactions to give hydrates — 1,1-diols, or geminal diols.

The formation of hydrates can be reversed by eliminating water from the diol.

At equilibrium, the amount of hydrate formed decreases with increasing alkyl groups on the carbonyl carbon.

Favorable steric and electronic factors associated with the alkyl groups cause the equilibrium to lie on the side of the carbonyl compounds.

Under neutral conditions, the rate of gem-diol formation is very slow, owing to the poor nucleophilicity of water. The reaction, however, can be accelerated using acid or base catalysts.

In acid catalysis, the hydronium ion protonates the carbonyl oxygen to generate a strongly electrophilic carbonyl carbon. This carbon is then attacked by water, forming an oxonium cation. Finally, deprotonation by another water molecule gives a gem-diol.

Under basic conditions, the stronger hydroxide nucleophile first adds to the carbonyl carbon, generating an alkoxide intermediate. A water molecule then protonates this intermediate to form the gem-diol.