Question

Following are the structure of D-Glucose and D-Galactose.


Which of the following statements are correct about the compounds?

• A. They are diastereomers
• B. Both are component of lactose
• C. They are C-4 epimer
• D. Both are optically active

Answer 1

Answer: (A), (B), (C), (D)

Both are optically active

Diastereomers are molecules that have 2 or more stereogenic centers and differ at some of these centers with respect to absolute configurations.
Glucose and galactose are aldohexoses. Aldohexoses have four asymmetric carbons, so there are 16 different optical isomers. So D-glucose and D-galactose are diastereomers. They have the same connectivity but are different optical isomers that are not enantiomers. both are optically active.

In lactose, there is a glycosidic bond $[1,4]$ between D-glucose and D-galactose where $C1$ of galactose is connected with $C4$ of glucose. It is a disaccharide and is found in milk. It is a reducing sugar because it has hemiacetal group. thus both
D-glucose and D-galactose are component of lactose.
When there is a change in configuration at only one chiral centre in a molecule, this pair is called as epimers. Remaining positions are same in the two molecules.$C-1$ epimer is known as anomer.​ Hence, option (a,b,c,d) are correct.