An oxygen-based nucleophile, like water, can undergo addition reactions with aldehydes and ketones. The reaction leads to the formation of hydrates, also referred to as 1,1-diols or geminal diols.

The formation of hydrates is a reversible reaction. Hydrate formation is influenced by steric and electronic factors accompanying the alkyl substituents on the carbonyl group: The rate of hydrate formation increases with a decrease in the number of alkyl groups attached to the carbonyl carbon. Hence, with simpler aldehydes, like formaldehyde, almost all aldehyde gives the corresponding hydrate. The formation of hydrates is highly favored when electron-withdrawing groups are linked to the carbonyl carbon.

The nucleophilic addition of water to the carbonyl carbon is very slow under normal conditions. However, the rate can be significantly enhanced by acidifying or basifying the reaction medium. An acid catalyst protonates the carbonyl oxygen, thereby increasing the electrophilicity of the carbonyl carbon. The nucleophilic addition of water leads to the formation of an oxonium cation, which loses a proton to form the geminal diol product.

On the other hand, the carbonyl carbon is directly attacked by the hydroxide nucleophile using a base catalyst. This addition generates an alkoxide ion, which is protonated by a water molecule forming the addition product.