Illustrated Glossary of Organic Chemistry

Fehling's test: A chemical reaction used to determine if a molecule contains an aldehyde or a terminal α-hydroxy ketone. To perform the test one adds Fehling's solution (which is blue due to the presence of Cu2+) to the unknown. If the unknown contains an aldehyde the Cu2+ is reduced to Cu+ which forms a red Cu2O precipitate. A terminal α-hydroxy ketone also gives a positive test because the Fehling's solution oxidizes a terminal α-hydroxy ketone to an aldehyde.


 Fehling's solution is blue due to dissolved Cu2+. The solution is homogeneous. There is no precipitate.

Add glucose

Addition of glucose causes reduction of Cu2+ to red Cu2O, forming a precipitate at the bottom of the test tube.