Why all monosaccharides are reducing sugar but not all disaccharides?

Question

Why all monosaccharides are reducing sugar but not all disaccharides?

Answer 1

because they should have free aldhyde or ketone groups to be reducing and if they react with their anomeric carbons(C1 in glucose and C2 in fructose) the resulting disaccharides won't be reducing forexample sucrose is D.Glc (1_>4)D.Frc so it's not a reducing sugar.

Answer 2

See, the property of 'reduction' by the monosaccharides is 'cuz of the presence of free aldehydic or ketonic grps in them (you have to know the general formula of say glucose or fructose to remember that).

But, not all disaccharides have free aldehyidc grps. Example , Sucrose is formed from Glucose and fructose in such a way that the reducing grps of both of them do not remain free. Hence, its a non-reducing sugar.

Examples of reducing sugars could be lactose and maltose.