Question

Which of the following are examples of Epimers?

• A. Glucose and Ribose
• B. Glucose and Galactose
• C. Galactose, Mannose and Glucose
• D. Glucose, Ribose and Mannose

Hint:

Mnemonic: "G-M-2" (Glucose/Mannose differ at C-2) and "G-Ga-4" (Glucose/Galactose differ at C-4). Epimers must have the same chemical formula!

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Epimers are a specific type of diastereomer (stereoisomer). Two sugars are called epimers if they differ in configuration around only one specific chiral carbon atom. All other chiral centers in the molecules must have identical configurations.

Step 2: Detailed Explanation:
Let's compare the structures of the most common hexoses:
- D-Glucose is the standard aldohexose.
- D-Galactose is identical to D-glucose except at the C-4 position, where the hydroxyl group (-OH) is on the left (in Fisher projection) instead of the right. Thus, Glucose and Galactose are C-4 epimers.
- D-Mannose is identical to D-glucose except at the C-2 position, where the hydroxyl group is on the left instead of the right. Thus, Glucose and Mannose are C-2 epimers.
Looking at the options: Option 1 is incorrect because Ribose is a pentose and Glucose is a hexose; they cannot be epimers as they have different numbers of carbon atoms. Option 2 provides a classic, universally recognized pair of epimers. Option 3 lists three hexoses that are all related via epimerization (Glucose is the central hub), which is also a very strong answer in many competitive contexts. However, usually, when a single pair is sought, Glucose and Galactose (or Glucose and Mannose) is the textbook example. Given the structure of the question, the pair in Option 2 is the definitive answer.

Step 3: Final Answer:
Glucose and Galactose are C-4 epimers, differing only in the orientation of the hydroxyl group at the fourth carbon.