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The Fehling's trial serves to detect an aldehyde group (-CHO) in aldehydes and reducing sugars.

To carry out the Fehling test, two solutions are used as detection reagents, which are referred to as "Fehling I" and "Fehling II" according to Hermann Fehling.

The light blue Fehling's solution I is a dilute copper (II) sulfate solution (70 g copper sulfate pentahydrate (CuSO4 · 5 H2O) fill up to 1 L water). The colorless one Fehling's solution II is an alkaline potassium sodium tartrate solution (100 g sodium hydroxide (NaOH) in approx. 500 mL water (H.2O) and add 340 g of potassium sodium tartrate tetrahydrate (KNaC4H4O6 · 4 H2O) and fill up to 1 L). The addition of 150 ml of the purest glycerine before filling with water extends the shelf life of a self-made solution. After combining the same volumes of both Fehling's solutions, the Fehling's reagent has a characteristic dark blue color due to the complex formation of the Cu (II) ions with the tartrate ions. After adding the test substance, the copper (II) ions are first reduced to yellow copper (I) hydroxide (CuOH) and then rearranged to copper (I) oxide (Cu2O).

Aldehydes are oxidized to carboxylic acids after the addition of Fehling's reagent, while copper sulfate (CuSO4) to copper (I) oxide (Cu2O) is reduced and precipitates out as a red-brown precipitate. With prolonged heating or with simpler aldehydes such as formaldehyde or acetaldehyde, elemental copper can also be formed.

The Fehling test - a redox reaction

Since the sample substance is oxidized by reducing the copper (II) ions, the overall reaction, as with all redox reactions, can be broken down into an oxidation and reduction reaction. For the sake of simplicity, the following example does not take into account that the copper ions are actually present in a complex with tartrate ions (copper tartrate):

Oxidation:

Reduction:

Redox reaction:


Since the reaction takes place in an alkaline environment, the resulting carboxyl group is deprotonated by hydroxide ions to form the carboxylate group in the sense of an acid-base reaction:


It should be mentioned at this point that ketones are not oxidized by Fehling's solution. The detection therefore allows a distinction to be made between a ketone and an aldehyde. An exception are saccharides, e.g. fructose (ketone), which can also be present in aldehyde form due to intramolecular proton migration in an alkaline medium (tautomerism).


Further detection reactions for aldehydes:

Category: Detection reaction