Aldehydes and ketones will react with water to form a hydrate. These are sometimes referred to as geminal diols or simply gem-diols. While this reaction is not particularly useful synthetically, gem-diols are formed as intermediates in some reactions. This reaction also serves as a model for understanding nucleophilic addition reactions.
In general, the rate of hydration is rather slow, unless it is catalyzed by acid or base.
Protonation of the ketone/aldehyde activates it for nucleophilic attack by water. The alkoxonium ion is subsequently deprotonated. Notice how the acid is regenerated, it's catalytic in acid.
Hydroxide ions are a much better nucleophile than water. The hydroxide ion attacks the ketone/aldehyde. In this case an alkoxide intermediate is formed, that is subsequently protonated by a water molecule.
Both sterics and electronics affect the electrophilicity of aldehydes/ketones and the position of equilibrium in hydration. This is a good measure of aldehyde/ketone reactivity.
|Khyd||% Conversion to Hydrate|