Question

Fehling's test is not positive in case of

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

Answer 1

The correct option is B Sucrose

Sucrose is formed by the glycosidic linkage between glucose and fructose. The bond is formed between the between C${}_1$on the glucosyl subunit and C${}_2$ on the fructosyl unit. It has no free aldehyde or keto group so they are known as non- reducing sugar. The anomeric carbon of glucose and fructose forms the glycosidic bonds. The non-reducing sugar is not able to give a positive test for Fehling's test. During Fehling's test when free aldehyde is present it is oxidized by bistartratocuprate (II) complex to a carboxylate anion and reduces the copper (II) ions of the complex to copper (I) ions. Red copper (I) oxide then precipitates and gives the positive test.

So, the correct answer is option B.