W4L8: Nucleophilic addition reactions: the basics

Aldehydes and ketones

Aldehydes and ketones contain a highly polar C=O bond

• Oxygen is very electronegative, and it draws electron density towards itself, giving a partial positive charge down on the carbon and a partial negative charge up on the oxygen.

• That means that carbonyls can act as nucleophiles or as electrophiles.

• The oxygen end, being negative, is nucleophilic, and therefore also basic

Nucleophilic addition to carbonyls

Nucleophiles can add to aldehydes and ketones

Neutral nucleophiles

• You can get addition of water, to form a geminal diol, or you can get addition of an alcohol to give a hemiacetal, where you have an alkoxy group instead of this hydroxyl

• A hemiacetal is not just an alcohol plus an ether – its reactivity is different to either of those species

• Addition of a thiol gives the analogous hemithioacetal, and addition of an amine gives an aminal

• Added the nucleophilic atom to the carbon, and added a proton to the oxygen.

Neutral nucleophiles: mechanism

Base catalysis: mechanism

Base catalysis

• Base catalysis allows us to reach equilibrium much faster than we would in the neutral reaction, and it works for all the species you met previously except the aminal

• That's because the other reactants are relatively easy to deprotonate under base catalysis, but amines are not - their anions are too unstable

Acid catalysis: mechanism

faster

Acid catalysis: further reaction