Fehling's test: practice and mechanism
Fehling's solution oxidises aliphatic aldehydes to the carboxylic acid (Cu²⁺ becomes red Cu₂O). Ketones do not react.
Example
Aliphatic aldehydes reduce copper(II) to red copper(I) oxide and are themselves oxidised to the carboxylic acid. Ketones do not react. Aromatic aldehydes barely give Fehling's test, unlike the Tollens test.
Mechanism step by step
- 1
Fehling's solution contains Cu²⁺ as a tartrate complex in sodium hydroxide (deep blue).
- 2
On warming Cu²⁺ oxidises the aldehyde to the carboxylate and is itself reduced to Cu⁺.
- 3
Cu⁺ precipitates as brick-red Cu₂O; acidification gives the carboxylic acid.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which reagent achieves this transformation?
Typical mistakes
Often confused with: Oxidation of aldehydes (Tollens test) · Reduction with sodium borohydride
Frequently asked questions
How does it differ from the Tollens test?
Tollens (Ag⁺, silver mirror) reacts with all aldehydes, including aromatic ones. Fehling (Cu²⁺, red precipitate) practically only with aliphatic aldehydes.
Why does glucose give a positive test?
Glucose is partly present in equilibrium as the open-chain form with a free aldehyde group. It is a reducing sugar.