Reactivity

One of the most common reactions of an aldehyde or ketone is the attack of an electron-rich species on the electron-deficient carbonyl carbon.

Lowest Unoccupied Molecular Orbital (LUMO) of formaldehyde, ready to accept electrons from the Highest Occupied Molecular Orbital (HOMO) of an electron-rich species. The attack takes place at the carbon, which has the larger orbital coefficient.

Animation of the addition of water to formaldehyde — Water addition to formaldehyde

The planar, sp²-hybridized carbon of formaldehyde is attacked by the electron cloud of the oxygen of water. The reaction proceeds through a transition state in which the carbon becomes pyramidal, changing its hybridization to sp³.

The nucleophiles that can add to aldehydes and ketones are very varied, with different synthetic consequences.