Attacking C=O with water

The carbon NMR of formaldehyde in water gives a single peak at 83 - which isn't in the expected 150 - 200 C=O region. This is because in water it converted to a 1,1 diol, also called a hydrate:

Which is overall an equilibrium:

Like the cyanohydrin reaction, this equilibrium is effected by steric effects. The R group repel as the bond angles are reduced from 120 to 109 degrees, they also block nucleophillic attack by water. This explains why aldehydes are usually more hydrated than ketones, especially formaldehyde.

Hydration can also be increased by using electron withdrawing groups, which polarize the C=O making it more prone to attack by water:

Carbonyls on three and four-membered rings are also especially hydrated, because they reduce relative ring strain:

The features described which increase hydration are the same ones which determine how electrophillic a C=O is. Dry cyclopropane is very electrophillic for example.

The hydrated form of chloral - known as chloro hydrate - is a sedative. It is used in Agatha Christie novels and to prepare a "Mickey Finn" - useful because it dissolves in water and alcohol, and is easy to synthesize: