Question

Fehling’s solution is:
A. Acidified copper sulphate solution
B. Copper sulphate and sodium hydroxide + Rochelle salt
C. Ammoniacal cuprous chloride solution
D. Ammonical $AgN{O_3}$ solution

Answer 1

Fehling solution is a solution discovered by Hermann von Fehling in $1849$. It was developed to confirm the presence of reducing and non reducing sugars.

Complete step by step answer: Fehling solution is a very important reagent in the laboratory. The test is named as Fehling’s solution test which uses Fehling’s solution. The aldehydes and α-hydroxy ketones are oxidized by Fehling’s reagent to corresponding acids. The carboxylic acid thus generates a complex structure with Fehling's solution.
Fehling’s solution is a combination of two solutions named as Fehling’s A and Fehling’s B. Fehling’s solution A is a deep blue aqueous solution of copper (II) sulfate. Fehling’s solution B is a colorless aqueous solution of potassium sodium tartrate. Both the solutions are made strongly alkaline with potassium hydroxide.
The two solutions are not kept as a mixture because the complex formed is not stable. They are kept separately and the mixture is prepared just before the test to be attempted with the sample for investigation.
The active reagent is a bis(tartrate) complex of $C{u^{2 + }}$ which is the oxidizing agent in the solution. The tartrate ions serve as bidentate chelating ligands with the $C{u^{2 + }}$ ion.
The net reaction with of an aldehyde and Fehling’s solution is written as:
$RCHO + 2C{u^{2 + }} + 5O{H^ - } \to RCO{O^ - } + C{u_2}O + 3{H_2}O$
$RCHO + 2Cu{({C_4}{H_4}{O_6})_2}^{2 - } + 5O{H^ - } \to RCO{O^ - } + C{u_2}O + 4{C_4}{H_4}{O_6}^{2 - } + 3{H_2}O$
Hence option B is the correct answer.

Note: The Fehling’s test does not work with aromatic aldehyde. In benzaldehyde the aldehyde carbonyl is in conjugation with the benzene ring generating a double bond in between the carbon atoms. As this bond is strong to break so benzaldehyde does not react with Fehling’s solution.