Question

Fehling’s test is not positive in case of-
a. Lactose
b. Sucrose
c. Glucose
d. Fructose

Answer 1

As we all know that Lactose and sucrose are disaccharides but lactose is a reducing sugar while sucrose is not. Aldehydes present in sugars give positive Fehling’s test.

Complete answer:
Fehling’s test is a chemical test, basically used to differentiate aldehyde and ketone functional groups. Here in this question, it is being used as a test to differentiate reducing and non-reducing sugars. Out of all the given options, only sucrose will not give a positive test as it is a disaccharide and does not contain free aldehyde or ketone group, the rest three sugars will give positive test. Though lactose and sucrose are both disaccharides, lactose has reducing properties but sucrose does not. The sugars which act as reducing agents in the alkaline solution are called reducing sugars.

Fehling’s solution is prepared by adding together Fehling’s A and Fehling’s B reagents. This solution acts as a highly oxidising agent. The presence of copper gives the solution blue colour which on reaction with the aldehyde group, turns copper ions to copper oxide which appears as red precipitate. The sugars which have free aldehyde groups get reduced but the ones which lack aldehyde groups, give out a negative test. The reaction is as follows-
$RCHO + 2Cu^{2+} + 5OH^{-} \to RCOO^{-} + Cu_{2}O + 3H_{2}O$

Hence, the correct answer is option (B).

Note: Presence of aldehyde but not ketones is detected by this test. The test will turn out to be positive only when a free aldehyde group is present. The Fehling’s test does not detect presence of sugar, it can only detect sugars having aldehyde group.