Question

Glucose and mannose are:
A. anomers
B. epimers
C. ketohexoses
D. Disaccharides

Answer 1

Hint: Glucose and mannose are the type of stereoisomers in which the compounds differ from each other in configuration at the stereogenic centers. They have the same chemical formula.

Complete answer:
Glucose and mannose have the same chemical formula, that is ${C}_{6}{H}_{12}{O}_{6}$. The molecular structure of glucose and mannose respectively are:

     Glucose $\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,$ Mannose

Let us now look at each term given in the question.
(A) Anomers: Anomers are the diastereoisomers of cyclic sugars or similar molecules that differ in the configuration at the anomeric carbon. Now, since, glucose and mannose are not cyclic compounds, therefore, they are not anomers.
(B) Epimers: Epimers are the compounds in which the isomers have different configurations of atoms about one of the several asymmetrical carbon atoms present. They are a specific type of stereoisomers which have multiple stereocenters but differ from one another by the configuration of one of the stereogenic centers. In case of glucose and mannose, they differ from each other by configuration at the C-2 atom. And thus, they are epimers.

       Glucose $\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,$ Mannose

(C) Ketohexoses: As the name suggests, ketohexose is a hexose that contains ketone. It is a monosaccharide that has both a ketone and six carbons. But, glucose and mannose do not contain any ketone group in its structure. Therefore, they are not ketohexoses.
(D) Disaccharides: Disaccharides is the sugar that is formed when two monosaccharides are joined by glycosidic linkage. But we can see that glucose and mannose and single compounds. Therefore, they are not disaccharides.
Therefore, glucose and mannose are epimers.

Hence, the correct answer is option (B).

Note: Glucose and mannose are also known as aldohexoses. This is because they contain an aldehyde group. These occur naturally as monosaccharides in fruits and in glycoproteins of algae and fungi.