Reactions of Aldehydes and Ketones

Distinguishing between aldehydes and ketones, Tollen's & Fehling's reagent, 2,4-DNPH, iodoform test

How can you test for aldehydes and ketones?

• Tollens' reagent

• Fehling’s solution

Tollens’ reagent is also known as what?

The ‘silver mirror’ test / ammoniacal silver nitrate solution.

State how Tollens’ reagent is made.

• Add silver nitrate (AgNO₃) to a test tube.

• Add a few drops of sodium hydroxide (NaOH).

• Add ammonia.

How can Tollens’ reagent be used to distinguish between an aldehyde / ketone?

• Add Tollens’ reagent to test with aldehyde / ketone.

• Heat in a water bath.

If an aldehyde is present, a silver mirror forms inside the tube.

Ketones do not form a silver precipitate.

How is Fehling’s solution used to distinguish between an aldehyde / ketone?

• Add blue Fehling’s solution (containing Cu²⁺ ions) to a test tube containing aldehyde / ketone.

If an aldehyde is present, a brick-red precipate is formed.

The solution remains blue with ketones.

How do you test for the C=O group in an aldehyde / ketone?

Using 2,4-DNPH.

The reaction that takes place is a nucleophilic addition-elimination reaction.

Describe how 2,4-DNPH is used to test for carbonyl compounds.

• A compound with a C=O group will react to give an orange-red precipitate.

• The precipitate is then filtered, purified and dried.

• The melting point of the solid is determined and used to identify the aldehyde / ketone.

What is the iodoform (triiodomethane) reaction?

It is used to identify:

• CH₃CO in ketones

• CH₃CH(OH) in alcohols

What are the reagents used in the iodoform reaction?

Iodine in aqueous NaOH (I₂/NaOH) or

KI in aqueous sodium chlorate(I) (KI/NaOCl)