Question

Which of the following pairs give positive Tollen's test?

• A. Glucose, sucrose
• B. Maltose, fructose
• C. Hexanal, acetophenone
• D. Fructose, sucrose

Answer 1

Answer: (B)

Maltose, fructose

Answer 2

Tollen's test is used to detect the presence of an aldehyde functional group. It gives a positive result (formation of a silver mirror) with aldehydes and reducing sugars. Ketones generally do not give a positive Tollen's test, except for α-hydroxy ketones like fructose, which can isomerize to aldehydes in the alkaline conditions of the Tollen's reagent.

• Maltose: A disaccharide composed of two glucose units. It has a free hemiacetal group on one of the glucose units, allowing it to open into an aldehyde form. It is a reducing sugar and gives a positive Tollen's test.
• Fructose: A ketohexose (contains a ketone group). In the alkaline conditions of Tollen's reagent, fructose can isomerize to glucose and mannose (both aldoses) through an enediol intermediate (Lobry de Bruyn-van Ekenstein transformation). These aldoses then react with Tollen's reagent. Therefore, fructose gives a positive Tollen's test.