Aldehydes are oxidised by Fehling’s solution and in this reaction, red coloured cuprous oxide is formed.
In Fehling’s solution the reaction between copper(II) ions and aldehyde is represented as;
RCHO + 2 Cu2+ + 5 OH− → RCOO− + Cu2O + 3 H2O
The result is positive if there is a formation of reddish-brown precipitate while the result is negative if there is no indication of such change.
Uses of Fehling’s Test
- It is used to determine whether a carbonyl group is an aldehyde or a ketone. Aldehydes tend to get oxidized and give a positive result. Ketones apart from alpha-hydroxy-ketones do not react.
- Fehling’s test is also used as a general test for monosaccharides where a positive result is obtained for aldose monosaccharides and ketose monosaccharides.
- It is also used in the medical field to determine the presence of glucose in urine. It helps to know whether the person is diabetic or not.