Fehling’s Solution

 

Definition: What is Fehling’s Solution?

Fehling’s solution, or Fehling’s reagent, is a chemical reagent that is used to distinguish between an aldehyde and a ketone, other than α-hydroxy ketone. Practically, it is used for the determination of reducing and non-reducing sugars that are present in carbohydrates. The test employed for this purpose is known as Fehling’s test. Fehling’s solution cannot be used for aromatic aldehyde [1-5].

Fehling’s Solution

The history of the test goes back to 1849 when German chemist Hermann von Fehling developed the reaction.

Preparation of Fehling’s Solution

Fehling’s solution is prepared by combining two separate solutions: Fehling A and Fehling B. Fehling A is a blue-colored aqueous solution of copper (II) sulfate (CuSO4). Fehling B is a colorless aqueous solution of potassium sodium tartrate (KNaC4H4O6·4H2O, also known as Rochelle salt) in an alkaline base like sodium hydroxide (NaOH). The two solutions are individually prepared and later mixed to give Fehling’s solution, which is blue. In this final mixture, aqueous tartrate ions from the dissolved Rochelle salt bond to Cu2+ (aq) ions from the dissolved copper sulfate crystals as bidentate ligands giving a bistartratocuprate (II) complex. 

Principle of Fehling’s Test

The principle of Fehling’s test is similar to that of Benedict’s test. When aldehydes are added to Fehling’s solution, they are easily oxidized by the bistartratocuprate (II) complex. During this process, copper (II) ions get reduced to copper (I) ions leaving a red precipitate of copper (I) oxide (Cu2O). The presence of red precipitate indicates a positive result [6,7].

Fehling’s Test Reaction

Example of Fehling’s Test

Fehling’s Test Glucose

Result of Fehling’s Test

Fehling’s Test

Mechanism of Fehling’s Test [8]

Fehling’s Test Mechanism

Uses and Applications of Fehling’s Solution

Fehling’s solution is used to test for monosaccharides. The most important application is to detect reducing sugar like glucose. Excess of glucose in blood and urine can lead to diabetes. Fehling’s reagent is also used in the breakdown of starch to glucose syrup and maltodextrins, a polysaccharide used as a food additive.  

FAQs

1. Does benzaldehyde give Fehling test?

Ans. No. Fehling’s test cannot be used for aromatic alcohol.

2. Does formaldehyde give Fehling test?

Ans. Yes. Fehling’s test can be used for formaldehyde.

3. Why are Fehling’s solutions A and B kept separate?

Ans. Fehling’s solutions A and B are kept separate because if they are combined, the bistartratocuprate (II) complex that is formed will quickly degrade.

4. Which sugar does not reduce Fehling’s solution?

Ans. Sucrose does not reduce Fehling’s solution because it does not have a free aldehyde or ketone group.

5. Why is aldehyde more reactive towards nucleophilic addition reaction than ketone?

Ans. Aldehyde is more reactive towards nucleophilic addition reaction than ketone because of its stereochemistry and electronic properties. Unlike ketone, aldehyde has single hydrogen on one side of the carbonyl functional group, which makes it easier for a nucleophile to attack.