0Class 12 Chemistry – Tollens’ and Fehling’s Tests (Aldehydes)
1. Introduction
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Aldehydes contain the –CHO group and can be distinguished from ketones using special tests.
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Tollens’ test and Fehling’s test are classical tests for aldehydes.
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Both tests are based on the oxidation of aldehydes.
2. Tollens’ Test
Principle
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Tollens’ reagent contains a silver-based complex.
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Aldehydes are oxidized, while the reagent is reduced, producing silver deposition.
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Ketones generally do not react in this test.
Observation
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Positive result: Formation of a shiny silver mirror on the inner surface of the test tube.
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Use: Distinguishes aldehydes from ketones.
3. Fehling’s Test
Principle
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Fehling’s solution contains copper ions in complex form.
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Aldehydes reduce Cu²⁺ ions to Cu⁺, producing a colored precipitate.
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Ketones generally do not react under normal conditions.
Observation
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Positive result: Formation of a brick-red precipitate.
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Use: Confirms the presence of an aldehyde functional group.
4. Key Points to Remember
| Test | Reagent Type | Observation for Aldehydes | Ketones |
|---|---|---|---|
| Tollens’ Test | Silver complex | Shiny silver mirror | No reaction |
| Fehling’s Test | Copper complex | Brick-red precipitate | No reaction |
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Both tests oxidize aldehydes while leaving ketones largely unaffected.
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These tests are classical qualitative methods used in laboratories.
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Applications:
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Detecting aldehyde functional groups in organic compounds.
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Identifying reducing sugars (they have an aldehyde group).
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