3.8: Aldehydes and Ketones

What are the chemical tests used to distinguish between aldehydes and ketones?

• Fehling’s solution: Blue solution produced red precipitate if aldehyde present

• Tollen’s: Silver mirror produced on surface if aldehyde present

What are aldehydes and ketones reduced to?

• Aldehydes: primary alcohols

• Ketones: seccondary alcohols

What is the reducing agent and name of the mechanism used to reduce aldehydes and ketones?

• Sodium tetrahydroborate (NaBH4)

• Nucleophilic additon

Outline the mechanism that takes place in the reduction of aldehydes and ketones

• Nucleophilic addition

• Lone pair on H- ion attacks carbon attached to =O

• Double bond breaks

• Lone pair on now negative O- attacks H+

• Produces alcohol

Outline the mechanism that takes place in the reduction of aldehydes and ketones by KCN followed by a dilute acid

• Nucleophilic addition

• Lone pair on C in CN attacks carbon attached to =O

• Double bond breaks

• Lone pair on now negative O- attacks H+ from the acid

• Produces alcohol

How is the reduction of aldehydes and ketones written in an equation?

reactant + 2[H] → product

How are hydroxynitriles produced?

Aldehydes and ketones undergo nucleophilic addition with KCN- followed by dilute sulphuric acid

Why does the reduction of aldehydes/ketones with KCN- produce a racemic mixture (and a mixture of enantiomers)?

• C=O is planar

• CN- can therefore attack from either above or below the plane of the molecule

What are the hazards of using KCN?

It can release hydrogen cyanide gas which is highly toxic