What is the test shown by aldehydes with Fehling's solution?

Aldehydes are oxidised by Fehling’s solution and in this reaction, red coloured cuprous oxide is formed.

Explanation

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.

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