Question

Which of the following carbohydrates is a NON-reducing sugar and therefore gives a NEGATIVE Benedict’s (and Fehling’s) test?

• A. Sucrose
• B. Maltose
• C. Lactose
• D. Glucose

Hint:

Which disaccharide locks BOTH of its anomeric carbons in the glycosidic bond?

Answer 1

The Correct Option is A

Step 1: Understand the basis of Benedict’s test.
Benedict’s test detects reducing sugars - carbohydrates with a free aldehyde or ketone group (a free anomeric carbon) that can reduce cupric (Cu²⁺) ions to cuprous oxide, giving a brick-red precipitate. A sugar with no free reducing group gives a NEGATIVE test.

Step 2: Examine each sugar.
Sucrose is glucose linked to fructose through an alpha-1 to 2 glycosidic bond that joins both anomeric carbons. Because both reducing groups are tied up in the glycosidic linkage, sucrose has no free reducing group and is a non-reducing sugar - Benedict negative.

Step 3: Why the others are positive.
Maltose (glucose-alpha-1,4-glucose) and lactose (galactose-beta-1,4-glucose) each retain one free anomeric carbon and are reducing sugars. Glucose is a free monosaccharide with an aldehyde group and is strongly reducing. All three give a positive Benedict’s test.

Key fact: Sucrose is the classic non-reducing sugar (both anomeric carbons engaged) and gives a negative Benedict’s test, while the rest reduce.