Addition of Water - Gem Diols

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.

Acid-catalyzed Hydration

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.

Base-catalyzed Hydration

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.

Factors Effecting Hydration

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
	22,000	~100
	41	99.96
	0.018	50
	0.0041	19
	0.000025	0.14