In disaccharides, if the reducing groups of monosaccharides i.e. aldehydic or ketonic groups are bonded, these are non-reducing sugars. Which of the following disaccharide is a non-reducing sugar?

Reducing sugars are sugars that are capable of acting as a reducing agent because they have a free aldehyde or a free ketone group.

Non-reducing sugars are sugars that are not capable of acting as a reducing agent because they don’t have a free aldehyde or a free ketone group.

The sugar given in option (ii) is a non-reducing sugar because the reducing group of glucose and fructose are involved in the glycosidic linkage. Thus, this sugar lacks a free aldehyde or a free ketone group. In other words, the reducing groups of monosaccharides are involved in glycosidic bond in this disaccharide. Hence, option (ii) is correct.

Options (i), (iii) and (iv) are incorrect because they are reducing sugars as they have a free aldehyde or a free ketone group.