Question

Glucose and galactose are having identical configuration in all the positions except position:
A. C-3
B. C-2
C. C-4
D. C-5

Answer 1

Hint: Carbohydrates that differ in the location of the hydroxyl (i.e., -OH) group only at one position are known as epimers. So, glucose and galactose are epimers as they have identical configuration at all the positions except for one position.

Complete answer:
Option (A) is incorrect. Glucose and galactose differ in only one position i.e., C-4 but not C-3.
Option (B) is incorrect. Glucose and mannose are epimers which, except in position C-2, have an identical configuration in all positions.
Option (C) is correct. Glucose and galactose are monosaccharides that differ from one another only at position C-4. Thus, they are epimers that have an identical configuration in all the positions except in position C-4.
Option (D) is incorrect. Glucose and galactose are epimers that do not differ in position C-5 but differ in position C-4.

Hence, the correct answer is option (C).

Additional information:
Stereoisomers are the type of isomers in which the arrangement of atoms in space is different but the order of the atoms in two molecules is the same. There are two types of stereoisomers i.e., enantiomers and diastereoisomers.
Enantiomers: are the stereoisomers that are relevant to each other by reflection which means that they are non-superimposable mirror images of one another.
Diastereoisomers: are the stereoisomers that do not form mirror images of each other.

Note: Epimers are diastereomers that have opposite configuration only in one position and the rest of the configuration is identical. They possess the same physical properties. Glucose and galactose differ at only one position i.e., C-4. Glucose is blood sugar i.e., an immediate source of energy for cellular respiration while galactose is a sugar in milk and yogurt.