Propanal and Fehling's sample

Experiment - Fehling's reagent with propanal

The experiment "Propanal and Fehling-Probe" works with different reagents that are mixed with one another. An assessment of the color change of the mixture then takes place.

• Two different Fehling solutions (Fehling I and Fehling II) as well as propanal and sulfuric acid are used in the experiment.
• In order to clarify the change in color after the reaction, a comparison solution is usually also used that contains a substance that the experiment does not work with and no color change takes place. Acetone is often used for this purpose.
• The sample with the propanal shows a strong change in color; initially the solution turns dark blue before it takes on a light gray and mud green tint. At the end the solution looks cloudy red.
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• The acetone solution, on the other hand, shows no change in color.


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Interpretation - this is why propanal reacts with Fehling's test

The detection of propanal with the Fehling's probe is used to detect a specific functional group, the aldehydes.

• Since the propanal is an aldehyde, the evidence here is positive. Propanoic acid is formed in the reaction. The Fehling mixture contains copper ions.
• Propanal has a reducing effect on copper (II) ions, so that in addition to propanoic acid, water and copper (II) oxide are also formed in the end product:
• 2 Cu²⁺ + C₂H₅CHO + 4OH^- → Cu₂O + C₂H₅COOH + 2H₂O
• The red-brown precipitate is copper oxide.
• Since acetone does not belong to the aldehydes, but to the ketones, the proof is negative here.

Please note: Chemical experiments may only be carried out by qualified personnel. As a layperson, you should never carry out such experiments on your own!