nucleophilic addition

Nucleophilic addition is the signature reaction of aldehydes and ketones, and the single most important reaction of this whole field. In one move, a carbon-oxygen double bond (C=O) becomes a carbon-oxygen single bond, and two new groups stick onto what used to be the flat carbonyl: a nucleophile lands on the carbon and something (often a hydrogen) ends up on the oxygen.

Here is the mechanism in plain steps. The carbonyl carbon is electron-poor (delta-plus), so an electron-rich nucleophile (Nu) attacks it. As the new C-Nu bond forms, one pair of electrons from the C=O double bond is pushed up onto the oxygen, which becomes negatively charged (an alkoxide). The carbon, once flat and trigonal, is now four-bonded and tetrahedral; this is the tetrahedral intermediate. Finally the negatively charged oxygen picks up a proton (from acid or from water) to become a neutral hydroxyl (O-H). Net result: Nu and H have added across the C=O double bond.

Almost every named reaction in this chapter is a variation on this one theme: only the nucleophile changes. Water gives hydrates, alcohols give hemiacetals and acetals, amines give imines, cyanide gives cyanohydrins, hydride (H-) gives alcohols, and carbon nucleophiles like Grignard reagents build new carbon-carbon bonds. Many of these additions are reversible equilibria, while others (like hydride or Grignard) effectively go to completion.