Identify Aldehyde from Ketone - Oxidation of aldehyde

Both aldehyde and ketone contain

Active group of aldehyde is and ketone's active group is

Differences of aldehyde and ketone

Aldehyde and ketone both have similar and difference chemical properties. Aldehyde can be oxidized to carboxylic acid easily while ketone are not.

Due to different active groups, aldehyde and ketone indicate different reactions for same reagent. But sometimes they show same changes (like colour changes) to some reagents.

The best way to identify aldehyde from ketone is checking the oxidizing ability of aldehyde or ketone

Oxidation of aldehyde

Aldehyde are oxidized to carboxylic acids by both strong and mold oxidizing agents.

Strong oxidizing agents

Aldehyde are oxidized to alcohols by strong oxidizing agents and their colour changes will notify strong oxidizing agents are being reduced.

  • H+ / KMnO4: purple colour is changed to light green or colourless.
  • H+ / K2CrO4: yellow colour is changed to green
  • H+ / K2Cr2O7: orange colour is changed to green

Mild strong oxidizing agents

  • Tollen's Reagent
  • Fehlingh's reagent

Aldehyde and tollen's reagent

Tollens reagent is prepared by dissolving AgOH white precipitate in ammonia solution.

When aldehyde is oxidized to carboxylic acid, Ag+ is reduced to Ag which is deposited in the bottom of the test tube. Deposited Ag is shown as a silver mirror.

But we cannot identify formic acid and aldehyde from tollen's reagent.

Below table shows some experiments can be done to identify aldehyde and ketone

Experiments to identify aldehyde and ketones

separate aldehyde and ketone