The Fehling’s test is another classical test used to detect and distinguish certain types of aldehydes.
Definition
Fehling’s test uses Fehling’s solution, which is a mixture of copper(II) sulfate solution (deep blue) and an alkaline solution of sodium potassium tartrate (Rochelle salt).
Principle
- Aldehydes can reduce the blue copper(II) ions in Fehling’s solution to red copper(I) oxide, which precipitates out.
- Ketones usually do not react (except some special cases like α-hydroxy ketones).
- Among aldehydes, aliphatic aldehydes give a positive test, while most aromatic aldehydes do not.
Observation
- On heating with Fehling’s solution, an aldehyde produces a brick-red precipitate of copper(I) oxide.
- If no reaction occurs, the solution remains blue.
Uses
- To distinguish aldehydes from ketones.
- To differentiate between aliphatic and aromatic aldehydes.
- To test for reducing sugars (like glucose), which also give a positive result.