Which of the following is non-reducing sugar?

Correct option is B

The correct answer is (b) Sucrose

Reducing vs. Non-Reducing Sugars

• A reducing sugar has a free aldehyde or ketone group capable of reducing mild oxidizing agents (e.g., Benedict’s reagent or Fehling’s solution).
• A non-reducing sugar lacks this free aldehyde or ketone group and does not react with these reagents.

Sucrose:

• Sucrose (table sugar) is a disaccharide composed of glucose and fructose linked via a glycosidic bond that does not allow a free aldehyde or ketone group.
• Hence, sucrose is a non-reducing sugar.

Other Sugars:

• Maltose, Lactose, and Cellobiose all have free aldehyde or ketone groups in their structures (after ring opening), making them reducing sugars.

Additional Information:

Reducing Sugars

• Definition: Sugars that can donate electrons to mild oxidizing agents (like Benedict’s or Fehling’s solution), typically due to a free aldehyde (–CHO) or ketone (–CO) group once the sugar ring opens in solution.
• Common Examples:
  • Glucose (monosaccharide)
  • Fructose (monosaccharide)
  • Maltose (disaccharide of two glucose units)
  • Lactose (disaccharide of glucose + galactose)
  • Cellobiose (disaccharide of two glucose units linked differently than in maltose)

Non-Reducing Sugars

• Definition: Sugars in which the glycosidic bond involves the aldehyde or ketone group, leaving no free reactive group to reduce other chemicals.
• Example:
  • Sucrose is the classic example because its anomeric carbons (from both glucose and fructose) are involved in the glycosidic linkage, preventing it from opening into a free aldehyde or ketone in solution.

Testing Reducing Sugars

• Benedict’s Test / Fehling’s Test:
  • If a sugar is reducing, the blue solution of copper(II) ions is reduced to a brick-red precipitate of copper(I) oxide.
  • Sucrose does not show this positive test unless it is hydrolyzed first.

Hydrolysis of Non-Reducing Sugars

• Sucrose can be hydrolyzed (broken down) by enzymes like invertase or by acidic conditions into glucose + fructose, both of which are reducing sugars.
• After hydrolysis, sucrose solutions will then test positive in Benedict’s or Fehling’s test.