Question

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

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

Answer 1

The correct option is B

Glucose, fructose

Tollens' reagent is a chemical reagent used to determine the presence of aldehyde, aromatic aldehyde, and alpha-hydroxy ketone functional groups. The reagent consists of a solution of silver nitrate and ammonia. Both glucose and fructose are reducing sugars, and they reduce Tollen’s reagent to metallic silver. Sucrose is a disaccharide of $\alpha$-D-glucose and $\beta$B-D-fructose. The glucose unit is a cyclic acetal, and the fructose unit is a cyclic ketal. Neither of these two units can ring open, and neither exists in equilibrium with the aldehyde form. Sucrose cannot react with Benedict's reagent and is therefore a nonreducing sugar.