Question

Regarding lactose, some statements are given below

A) On hydrolysis, lactose gives β-D-galactose and β-D-glucose

B) In lactose C₁ of β-D-galactose has acetal structure and C₁ of β-D-glucose has hemiacetal structure

C) In lactose molecule, β-D-galactose is a non-reducing unit and β-D-glucose is reducing unit

The correct statements are

• A. A, C
• B. A, B
• C. B, C
• D. Only A

Answer 1

The correct option is D

Only A

A simple way to test if the compound is α or β is to see the position of -OH group on carbon 1, which is the carbon to the right of O.

If the -OH group is above the ring, then it is β. If it is below the ring, it is α. In this case, both are above the ring so it is β. First statement is correct.



The one on the left is acetal and the right is hemiacetal. The difference is that there are two -OR groups on the carbon in case of acetal and there is one -OH group and one -OR group in the case of hemiacetal.

If you look at the C1 in galactose and glucose, you will find that both have an -OH group and an -OR group in the cyclic ring. So, both are acetals.

And finally, to see if a sugar is reducing you need to check if there is an -OH group on the C1 carbon. In both cases, we see that there is an -OH group on the C1 carbon. So both are reducing sugars.