IIT JEE Advanced 2009 Paper 2

Reducing sugars contain cyclic hemiacetal or hemiketal groups in equilibrium with the open chain form having a free –CHO or –C = O group. Non-reducing sugars contain stable acetal or ketal structures Their cyclic structures cannot be opened into an open chain form having a free carbonyl group. In X the glycosidic linkage is in between two anomeric carbon atoms while in Y it is only with one anomeric carbon, the other one is free. So, X is non-reducing while Y is reducing.