Question

How can you distinguish between Aldehydes and Ketones using the Tollen’s or Fehling’s test?

Hint:

Aldehydes give silver mirror (Tollen’s) and red precipitate (Fehling’s), while ketones generally give negative results.

Answer 1

Concept: Aldehydes and ketones both contain the carbonyl group, but aldehydes are easily oxidized while ketones are resistant to mild oxidation. This difference is used in qualitative tests like Tollen’s and Fehling’s tests.
Step 1: Tollen’s Test (Silver Mirror Test).

• Reagent: Ammoniacal silver nitrate (Tollen’s reagent).
• Aldehydes reduce Ag$^+$ to metallic silver, forming a shiny silver mirror on the test tube.
• Ketones generally do not give this reaction.

Observation:

• Aldehyde → Silver mirror formed
• Ketone → No reaction

Step 2: Fehling’s Test.

• Reagent: Fehling’s solution (alkaline Cu$^{2+}$ complex).
• Aldehydes reduce Cu$^{2+}$ to Cu$_2$O, forming a brick-red precipitate.
• Ketones usually do not react (except some $\alpha$-hydroxy ketones).

Observation:

• Aldehyde → Brick-red precipitate
• Ketone → No precipitate

Step 3: Reason for difference. Aldehydes contain a hydrogen atom attached to the carbonyl carbon, making them easily oxidizable. Ketones lack this hydrogen and are resistant to mild oxidizing agents.
Conclusion: Tollen’s and Fehling’s tests help distinguish aldehydes from ketones based on the ease of oxidation of aldehydes.