Question
Explain Fehling’s solution test.
✓
Answer
- Fehling’s solution is a mixture of Fehling’s solution $‘A \ ’$ containing $CuSO_4$ solution and Fehling’s solution $‘B \ ’$ containing sodium potassium tartrate $($Rochella salt$)$ in caustic soda $(NaOH)$ solution.
- When an aldehyde is heated with Fehling’s solution, the deep blue colour of the solution disappears and $Cu^{+2} ($cupric ion$)$ is reduced to $Cu^+$ ion a red precipitate of cuprous oxide, $Cu_2O$ is obtained while aldehyde is oxidised to a carboxylate ion.
- For example,
- This test is not given by ketones, since they cannot be oxidised by Fehling solution.
- Aromatic aldehydes are not oxidised by Fehling solution.
- Hence this test is used to distinguish between aldehydes and ketones.
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