Chemistry conceptual note on Tollens’ and Fehling’s tests for aldehydes, no equations

Class 12 Chemistry – Tollens’ and Fehling’s Tests (Aldehydes)

1. Introduction

• Aldehydes contain the –CHO group and can be distinguished from ketones using special tests.

• Tollens’ test and Fehling’s test are classical tests for aldehydes.

• Both tests are based on the oxidation of aldehydes.

2. Tollens’ Test

Principle

• Tollens’ reagent contains a silver-based complex.

• Aldehydes are oxidized, while the reagent is reduced, producing silver deposition.

• Ketones generally do not react in this test.

Observation

• Positive result: Formation of a shiny silver mirror on the inner surface of the test tube.

• Use: Distinguishes aldehydes from ketones.

3. Fehling’s Test

Principle

• Fehling’s solution contains copper ions in complex form.

• Aldehydes reduce Cu²⁺ ions to Cu⁺, producing a colored precipitate.

• Ketones generally do not react under normal conditions.

Observation

• Positive result: Formation of a brick-red precipitate.

• Use: Confirms the presence of an aldehyde functional group.

4. Key Points to Remember

• Both tests oxidize aldehydes while leaving ketones largely unaffected.

• These tests are classical qualitative methods used in laboratories.

• Applications:

  • Detecting aldehyde functional groups in organic compounds.

  • Identifying reducing sugars (they have an aldehyde group).