Glucose and galactose are having identical configuration in all the positions except position.
(a) C

Q: What exactly is an epimer?

An epimer is a specific type of diastereomer where two molecules differ in the spatial arrangement of atoms at exactly one stereogenic (chiral) center, while being identical at all others.

Q: Are glucose and galactose enantiomers?

No, enantiomers are non-superimposable mirror images of each other (they differ at *all* chiral centers). Glucose and galactose differ at only one chiral center (C-4), making them diastereomers, specifically epimers.

Q: How do I remember the structures of these sugars?

A common trick is to memorize the structure of D-glucose (Right-Left-Right-Right for the -OH groups from C-2 to C-5). Then remember that Mannose is the C-2 epimer and Galactose is the C-4 epimer.

Q: Why don't we consider C-1 or C-6?

In the open-chain form, C-1 is an achiral aldehyde carbon, and C-6 is an achiral primary alcohol carbon. Stereoisomerism only occurs at the chiral centers (C-2, C-3, C-4, and C-5).

Q: Does this difference affect their properties?

Yes! Even though they differ at just one carbon, they are distinct chemical compounds with different physical properties (like melting point and optical rotation) and biological functions.

Question

Glucose and galactose are having identical configuration in all the positions except position.

Options:

(A) C-3

(B) C-4

(C) C-2

(D) C-5

Sigma Percentile · Accepted Answer

### The Tale of Two Sugars

Let's look at the structures of D-glucose and D-galactose. These are two very common monosaccharides, and they look almost identical at first glance. Both are aldohexoses, meaning they are six-carbon sugars with an aldehyde group at the top. They even share the exact same molecular formula, $C_6H_{12}O_6$.

However, biology is incredibly specific. Even a tiny change in the spatial arrangement of atoms can turn one sugar into a completely different one with distinct properties. This brings us to the concept of epimers.

### What are Epimers?

When two sugars (or any diastereomers) differ in configuration at exactly **one chiral center**, we call them epimers. To figure out if glucose and galactose are epimers, and at which position, we need to scan their carbon chains from top to bottom and find that single point of difference.

### Scanning the Carbon Chain

Starting from the aldehyde group at the top (C-1), let's look at carbon two (C-2). In both D-glucose and D-galactose, the $-OH$ group is on the right side.

Now look at carbon three (C-3). In both molecules, the $-OH$ group is on the left side. So far, they are perfectly identical.

### The C-4 Difference

Now, focus on carbon four (C-4). This is where the magic happens. In D-glucose, the $-OH$ group is on the **right**. But in D-galactose, the $-OH$ group is on the **left**!

If you continue to C-5, both have the $-OH$ group on the right again. Therefore, C-4 is the *only* point of difference between these two molecules.

### Final Conclusion

Because they differ only at the C-4 position, glucose and galactose are classified as **C-4 epimers**. Thus, the correct option is **(b)**.

As a bonus fact for your exams: Do you know which sugar is the C-2 epimer of glucose? It's **Mannose**! Remembering these specific epimers (Glucose-Galactose at C-4, and Glucose-Mannose at C-2) is a high-yield concept for JEE and NEET.

Animated Solution for Chemistry - Organic Chemistry: Glucose and galactose are having identical configuration in all the positions except position.

Select Answer:

Visualized Solution

$Structures of D-Glucose and D-Galactose$

$Concept of Epimers$

$Comparing C-2 and C-3$

$Comparing C-4$

$Conclusion$

$Bonus: C-2 Epimer$

The Sigma Insight: Biomolecules

The Tale of Two Sugars

What are Epimers?

Scanning the Carbon Chain

The C-4 Difference

Final Conclusion

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