Question

L-Mannose is an epimer of:
A. D-Fructose
B. D-Glucose
C. D-Galactose
D. none of the above

Answer 1

Hint: Consider the number of carbon atoms found in L-mannose and also all of the options given here. Also consider the nature of the given options as aldoses or ketoses.

Complete answer:
Epimers are a specific type of stereoisomer that has multiple stereocenters but only differ from one another by the configuration at one of the stereogenic centers. By stereogenic center, we mean, the carbon atom that has four different atoms or groups of atoms bonded to it. The molecular formula and the functional groups found in epimers is the same.
We know that D-fructose is a ketose (having ketone group) sugar while L-mannose, D-glucose, as well as D-galactose are all aldose (having aldehyde group) sugars. Thus, we can directly rule out D-fructose as one of our answers.
In this case, we need to examine the structure of all the mentioned options to find the epimer of L-Mannose.
The structure of L-mannose is:

Here, we can see the aldose group at the leftmost end of the molecule.

The structure of D-glucose is:

Here, we can see the aldose group at the leftmost end of the molecule. We can also see that the position of the hydroxyl group attached to the second carbon from the left is different from the position seen in mannose, the rest of the molecule is the same.
The structure of D-galactose is:

Here, we can see the aldose group at the leftmost end of the molecule. Compare the structure of this molecule with the structure of L-mannose, you will see that the position of the hydroxyl group differs in two places. The second and the fourth carbons have hydroxyl groups that are in different positions than L-mannose.
From this, we see that in D-glucose and L-mannose, one of the stereocentres have a different stereochemistry but in D-galactose and L-mannose, 2 stereocenters have different stereochemistry. So according to the definition, L-mannose can be the epimer of only D-glucose.
So, we can say that L-Mannose is an epimer of D-Glucose. The correct answer is ‘ D-glucose’
So, the correct answer is “Option B”.

Note: We should know that Mannose is a C-2 epimer of glucose and a sugar molecule of the aldohexose series of carbohydrates. D-galactose is also a carbohydrate of the aldohexose series of carbohydrates but it is a C-4 epimer of glucose. Mannose is important in human metabolism, particularly in the proper glycosylation of native proteins.