Question

Which statements are correct?
(This question has multiple correct options)
A.Lactose is a disaccharide and is a reducing sugar
B. $\alpha - D$ – glucopyranoside has an acetal structure and is non-reducing sugar
C.Methyl $\alpha - D$ - glucopyranose has an acetal structure and is a non-reducing sugar
D. $\alpha - D$ – Glucopyranose has a hemiacetal structure and is a reducing sugar

Answer 1

Glucose will reduce Fehling's solution $[Cu(II) \to Cu(I)]\left[ {Cu\left( {II} \right) \to Cu\left( I \right)} \right]$ and Tollens’s reagent $[Ag(I) \to Ag(0)]\left[ {Ag\left( I \right) \to Ag\left( 0 \right)} \right]$ and it is thus classified as a reducing sugar.

Complete step by step answer:
A reducing sugar is any sugar that is capable of acting as a reducing agent because it has a free aldehyde group or a ketone group.
Glucose is reducing sugar because it can be oxidised by and thus reduce mild oxidising agents such as $C{u^{2 + }}$ or $A{g^ + }$ . All monosaccharides including glucose are reducing in nature.
In order for an oxidation to occur, the cyclic form must first ring-open to give the reactive aldehyde.
Hemi-acetal is a compound formed by the addition of an alcohol to an aldehyde or ketone. Any sugar that contains a hemi- acetal will be a reducing sugar. Any Glycosides which are acetal are not reducing sugars.
Lactose is a disaccharide sugar. It is also a reducing sugar composed of one molecule of D-galactose and one molecule of D-glucose found in milk joined by a $\beta$ – 1, 4-glycosidic bond (the bond from the anomeric carbon of the first monosaccharide unit being directed upward).
$\alpha - D$ – glucopyranoside has an acetal structure and is non-reducing sugar.
Methyl $\alpha - D$ -glucopyranose has acetal groups and hence is not a reducing sugar as the aldehyde group is not free, and is present in the form of acetal.
$\alpha - D$ -Glucopyranose has a hemiacetal structure and is a reducing sugar as they can be changed to open chain form with an aldehyde group.

Therefore, the correct answer is option (A), (B), (C) and (B).

Note: The glycosides which are acetal are not reducing sugar. Ketoses can also be reducing sugars because they can isomerise (a tautomerization) to aldoses.