The Fehling reaction or Fehling test is a method that allows to detect and, to some extent, quantify the reducing sugars in a sample. Its chemical properties are very similar to those of the Benedict reaction, differing only in the copper complex that participates in the oxidation of sugars.
The Fehling test is also used to distinguish between an aldehyde and a ketone; however, alpha-hydroxyketones give a positive response, as is the case with the monosaccharides ketoses. Thus, the aldoses (monosaccharides aldehydes) and ketoses, making up the reducing sugars, are oxidized to their respective acidic forms.
The image above shows Fehling’s reagent in the test tube on the left. Its bluish color is due to CuSO 4 · 5H 2 O dissolved in water, whose copper ions complex with tartrate anions, preventing copper hydroxide from precipitating in an alkaline medium.
After the reaction has elapsed in a hot bath at 60ºC and in the presence of aldehydes or reducing sugars, a brown precipitate forms, indicative of a positive test.
This precipitate is cuprous oxide, Cu 2 O, which can be weighed to determine how many reducing sugars or aldehydes were in the sample.
Fehling’s reagent preparation
Fehling’s reagent actually consists of a mixture of two solutions, A and B, in which the bistartratocuprate (II) complex is formed; this is the true active agent.
Fehling’s Solution A is an aqueous solution of CuSO 4 · 5H 2 O, to which a small amount of sulfuric acid can be added to help dissolve the bluish crystals. Depending on the volumes needed, 7 g or 34.65 g of the copper salt are dissolved, 100 mL or 400 mL, respectively, are transferred to a volumetric flask, and made up to the mark with distilled water.
This solution is light blue in color and contains Cu 2+ ions , which will be the reduced species when the Fehling reaction takes place.
Fehling’s solution B is a strongly alkaline solution of sodium potassium tartrate, also known as La Rochelle’s salt, in sodium hydroxide.
The formula of this salt is KNaC 4 H 4 O 6 · 4H 2 O, which can be written as HO 2 CCH (OH) CH (OH) CO 2 H, and 35 g of it are dissolved in 12 g of NaOH making up to 100 mL of distilled water. Or if more amounts of La Rochelle salt are available, 173 g are weighed and dissolved in 400 mL of distilled water with 125 g of NaOH, making up to 500 mL with distilled water.
The purpose of the strongly alkaline medium is to deprotonate the central hydroxyl groups OH of the tartrate, so that its oxygen atoms can coordinate with the Cu 2+ and establish the bistartratocuprate complex (II). This darker blue complex is formed when equal volumes of solutions A and B are mixed.
Once this is done, a 2 mL aliquot is taken and transferred to a test tube, to which 3 drops of the sample that we want to find out if it has an aldehyde or reducing sugar will be added. Then and finally, the properly supported test tube is placed in a hot water bath at 60ºC, and it is waited for the appearance of a brown precipitate indicative of a positive test.
In the upper image we have the structural formula of the bistartratocuprate complex (II). Each Cu 2+ ion in solution A complexes with two tartrates from solution B, preventing the copper hydroxide from precipitating due to the presence of OH – ions from the medium.
This complex could be written as Cu (C 4 H 4 O 6 ) 2 2− . Why has the negative charge changed from -6 to -2? This is because the image does not take into account the surrounding K + or Na + ions , which neutralize the negative charges of the carboxylate groups, -CO 2 – , at the ends of the complex.
Thus, Cu (C 4 H 4 O 6 ) 2 6− when surrounded by two pairs of K + and Na + , its charge remains as Cu (C 4 H 4 O 6 ) 2 2− , where in the center of the complex we have Cu 2+ .
What is the reaction that takes place when this complex comes in contact with an aldehyde, an aldose, or a ketosis? Ketose, in their cyclic conformation, oxidizes their anomeric carbon C-OH to CHO: an aldose, which then continues to oxidize to its acid form, COOH.
The following chemical equation shows the oxidation of aldehydes to carboxylic acids:
RCHO + 2 Cu (C 4 H 4 O 6 ) 2 2− + 5 OH – → RCOO – + Cu 2 O + 4 C 4 H 4 O 6 2− + 3 H 2 O
But, because the medium is strongly alkaline, we have RCOO – and not RCOOH.
The oxidized aldehyde, aldose, or ketose, RCHO, is oxidized as it acquires an extra bond with oxygen. On the other hand, Cu 2+ ions are reduced to Cu + (Cu 2 + O 2- ), the species being reduced. As the complex reacts and forms the red Cu 2 O precipitate , the tartrate ions are released and remain free in the medium.
Uses and examples
When an aldehyde or ketone is suspected, a positive Fehling’s reagent test indicates that it is an aldehyde. This is often very useful in organic qualitative tests. Any aldehyde, as long as it is aliphatic and not aromatic, will react and we will see the red precipitate of Cu 2 O.
The Fehling reaction makes it possible to quantify the amount of reducing sugars in the sample by weighing Cu 2 O. However, it is not useful to discern between an aldose or ketosis, since both give positive results. Sucrose is one of the few sugars that gives a negative result, the solution remaining bluish.
Glucose, fructose, maltose, galactose, lactose, and cellobiose, being reducing sugars, respond positively to Fehling’s reagent; and therefore, thanks to this method they can be detected and quantified. For example, the amount of glucose in the blood and urine has been quantified using Fehling’s reagent.
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