Question

Fehling’s solution is used for detection of
A) Glucose
B) Starch
C) All type of carbohydrates
D) Fats

Answer 1

Hint: Fehling's solution is used to decide whether an aldehyde or ketone is a carbonyl-containing compound. The Fehling test can be used as a standardised monosaccharide test. Fehling can be used to test for glucose in the urine, thereby predicting diabetes.

Complete answer:
Fehling's solution can be used to differentiate aldehyde from functional ketone groups. The substance to be measured is applied to the Fehling solution and the mixture is heated. Aldehydes are oxidised, giving a positive effect, but ketones do not react because they are hydroxy ketones. The bistartratocuprate (II) complex oxidises the aldehyde to a carboxylate anion, and the copper(II) ions of the complex are reduced to copper(I) ions. Red copper(I) oxide then precipitates out of the reaction mixture, indicating a positive outcome, i.e. the redox has occurred (this is the same positive result as the Benedict solution).

The net reaction between aldehyde and copper(II) ions in Fehling's solution can be written as
Follows:
Fehling's can be used for urinary glucose monitoring, so it is possible to classify diabetes.
The net reaction between an aldehyde and the copper(II) ions in Fehling's solution may be written as:
\[RCHO + 2C{U^{2 + }} + 5O{H^ - } \to RCO{O^ - } + C{u_2}O + 3{H_2}O\]
or with the tartrate included:
\[RCHO + 2Cu{\left( {{C_4}{H_4}{O_6}} \right)_2}^{2 - } + 5O{H^ - } \to RCO{O^ - } + C{u_2}O + 4{C_4}{H_4}{O_6}^{2 - } + 3{H_2}O\]

Hence the correct answer is option ‘A’.

Notes: The Fehling test is a general monosaccharide test, which produces favourable results for monosaccharides and ketosis. In addition, the Fehling test is used in the medical field to evaluate the presence of glucose in the urine. It allows us to know whether or not a person is diabetic.