Intermediate Topic 806 of 1102 Organic Chemistry
Fehling's Test
Aldehydes reduce blue copper(II) to red copper(I) oxide.
In-depth explanation of Fehling's Test
Fehling's test uses an alkaline solution of copper(II) ions complexed with tartrate, which is blue. Aliphatic aldehydes reduce these copper(II) ions to copper(I) oxide, a brick-red precipitate, while the copper is itself reduced. Aromatic aldehydes and ketones usually do not react under the test conditions, which helps distinguish aliphatic aldehydes from other carbonyl compounds. The test is widely used to detect reducing sugars.
Going deeper: Fehling's Test
Key ideas: Fehling A is aqueous copper sulfate and Fehling B is alkaline sodium potassium tartrate (Rochelle salt), mixed to give a deep blue Cu2+ complex. Warm with aliphatic aldehydes to obtain a brick-red precipitate of Cu2O. Benzaldehyde, ketones and aromatic aldehydes do not respond. Reducing sugars like glucose respond, and Benedict's reagent is the citrate analogue.
Common mistakes and connections: Students say ketones are always negative, but alpha-hydroxy ketones such as fructose do respond. Another error is expecting a red solution rather than a precipitate. Tartrate keeps Cu(OH)2 in solution in alkaline medium. Links to Tollens test, reducing sugars and distinguishing aliphatic from aromatic aldehydes.
Visualization of Fehling's Test
Aliphatic aldehydes reduce blue Cu2+ to a red Cu2O precipitate; ketones and aromatic aldehydes like benzaldehyde do not. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Benedict's solution, a related copper reagent, gives the same colour change with reducing sugars such as glucose.
Frequently asked questions about Fehling's Test
What is Fehling's Test?
Aldehydes reduce blue copper(II) to red copper(I) oxide.
Is Fehling's Test a beginner, intermediate or advanced topic?
Fehling's Test is a intermediate topic in Organic Chemistry. It builds on the vocabulary of beginner topics in this area.
What should I learn before Fehling's Test?
Study these earlier topics in Organic Chemistry first: Aldol Condensation, Cannizzaro Reaction, Tollens' Test.
What comes after Fehling's Test?
The next topics in Organic Chemistry are Grignard Reagents and Carboxylic Acids.
Can I practise Fehling's Test with quizzes?
Yes. The interactive lesson includes a self-check question on Fehling's Test, and the Organic Chemistry quiz draws on every topic in this area.
Where Fehling's Test fits in Organic Chemistry
Fehling's Test is topic 78 of 148 in Organic Chemistry.