Fehling’s Test

                    Principle                  

                                 Fehling's test differentiates between aldehydes and ketones. Aldehydes can be oxidized by Cu²⁺ in the presence of a strong base to form carbonic acids. Ketones cannot be oxidized by this reaction. When the Cu²⁺ oxidizes the aldehydes it is reduced to Cu⁺, and forms the compound Cu₂O, which is a reddish precipitate. That is how you know you have an aldehyde.

                            Fehling's reagent is usually formed by mixing CuSO₄, sodium citrate, and sodium carbonate. The carbonte is used to raise the pH, but doing so would normally form Cu(OH)₂, a precipitate. That is where the sodium citrate comes it---the citrate complexes the Cu2⁺ ions and prevents them from precipitating.

              Fehling's solution               

                    Fehling's solution is a chemical test used to differentiate between water-soluble aldehyde and ketone functional groups, and as a test for monosaccharides. The test was developed by German chemist Hermann von Fehling in 1849.

                    Fehling's solution is always prepared fresh in the laboratory. It is made initially as two separate solutions, known as Fehling's A and Fehling's B. Fehling's A is a blue aqueous solution of copper(II) sulfate, while Fehling's B is a clear solution of aqueous potassium sodium tartrate (also known as Rochelle salt) and a strong alkali (commonly sodium hydroxide).

                    Equal volumes of the two mixtures are mixed together to get the final Fehling's solution, which is a deep blue colour. In this final mixture, aqueous tartrate ions from the dissolved Rochelle salt chelate to Cu²⁺ (aq) ions from the dissolved copper(II) sulfate, as bidentate ligands giving the bistartratocuprate(II)^4- complex as shown below. The tartarate ions, by complexing copper prevent the formation of Cu(OH)₂ from the reaction of CuSO₄.2H₂O and NaOH present in the solution.

Procedure:

- To 1 mL of Fehling’s solution A (aqueous solution of CuSO4) add 1 mL of Fehling solution B (solution of potassium tartrate).

- Add 2 mL of the sugar solution, mix well and boil.

•  Try to see the red precipitate of cuprous oxide that forms at the end of the reaction.

Questions:

• Write the reaction(s) involved in Fehling’s Test.
• What is the function of tartrate?
• Some disaccharides such as maltose are reducing agents, whereas others, such as sucrose are not. Explain briefly by incluiding the structures of the sugars.