Question

Ketones do not reduce Fehling’s solution and Tollen’s reagent, while fructose containing ketonic group does. Why?

Hint:

Remember: Fructose is an exception because it tautomerizes to glucose/mannose in base, making it act like an aldehyde.

Answer 1

Step 1: General case of ketones.
Normally, ketones cannot reduce Fehling’s solution or Tollen’s reagent because they cannot be easily oxidized under mild conditions. Only aldehydes undergo such mild oxidation. Step 2: Special case of fructose.
Fructose is a ketohexose, but in alkaline medium it undergoes
tautomerization (Lobry de Bruyn–van Ekenstein transformation) to form glucose and mannose, both of which are aldehydes. \[ Fructose \;\xrightarrow{OH^-}\; Glucose + Mannose \] Step 3: Reaction with reagents.
Since glucose and mannose contain aldehyde groups, they reduce Fehling’s solution and Tollen’s reagent. Thus, fructose indirectly gives positive tests. Conclusion:
Ordinary ketones do not reduce Fehling’s or Tollen’s reagent, but fructose does due to tautomerization into aldoses in alkaline medium.