Aldehydes and ketones: nucleophilic addition reactions

Aldehydes and ketones contain a carbonyl group, C=O.

Oxygen is a very electronegative element and the C=O bond is highly polar.

Nucleophiles can attack the partially positively charged carbon atom.

A covalent bond forms between the nucleophile and the carbon atom.

A pair of electrons from the carbon–oxygen double bond moves to the oxygen atom.

A carbon–oxygen single bond forms and the oxygen atom gains a negative charge.

A covalent bond forms between the oxygen atom and a hydrogen ion, H +. This ion can come from the solvent, water.

A hydroxyl group forms in the final product.

This is a general mechanism for nucleophilic addition in carbonyl compounds.

Let's look at the mechanism of the reaction between a carbonyl compound and the cyanide ion, CN –.

Oxygen is a very electronegative element and the C=O bond is highly polar.

The cyanide ion has a lone pair of electrons on its carbon atom. It acts as a nucleophile and it can attack the partially positively charged carbon atom.

A covalent bond forms between the cyanide ion and the carbon atom.

A pair of electrons from the carbon–oxygen double bond moves to the oxygen atom.

A carbon–oxygen single bond forms and the oxygen atom gains a negative charge.

A covalent bond forms between the oxygen atom and a hydrogen ion, H +. This ion can come from the solvent, water.

A hydroxyl group forms.

The final product is a hydroxynitrile.

Let’s look at the reaction mechanism when the carbonyl compound is an aldehyde, ethanal.

The final product is 2-hydroxypropanenitrile. It contains an asymmetric carbon atom and shows optical isomerism.

Let’s look at the reaction mechanism when the carbonyl compound is a ketone, propanone. The final product is 2-hydroxy-2-methylpropanenitrile, which does not show optical isomerism.

Let's look at the mechanism of the reaction between an aldehyde, ethanal, and a reducing agent such as sodium tetrahydridoborate(lll), NaBH 4.

Oxygen is a very electronegative element and the C=O bond is highly polar.

The BH – ion provides the hydride ion, H –. This has a lone pair of electrons and it acts as a nucleophile. It can attack the partially positively charged carbon atom.

A covalent bond forms between the hydride ion and the carbon atom.

A pair of electrons from the carbon–oxygen double bond moves to the oxygen atom.

A carbon–oxygen single bond forms and the oxygen atom gains a negative charge.

A covalent bond forms between the oxygen atom and a hydrogen ion from water, the solvent.

A hydroxyl group forms.

The final product is a primary alcohol, in this case ethanol.

This is the reaction mechanism when the carbonyl compound is a ketone, propanone. The final product is a secondary alcohol, in this case propan-2-ol.