Problem 12 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following compounds will not show mutarotation? (a) \(\beta-\mathrm{D}(+)\) glucopyranose (b) \(\alpha-\mathrm{D}(+)\) glucospyranose (c) \(\beta-\mathrm{D}(+)\) galactopyranose (d) methyl- \(\alpha\)-D-glucopyranoiside

Step-by-Step Solution

Answer

Methyl-\(\alpha\)-D-glucopyranoiside will not show mutarotation.

Step 1: Understanding Mutarotation

Mutarotation is the change in optical rotation that occurs by the conversion of one anomer to another. It necessitates a free hydroxyl group at the anomeric carbon that can open to form an open-chain structure and is capable of converting from one anomer to another.

Step 2: Evaluate Each Compound

(a) \( \beta-\mathrm{D}(+) \) glucopyranose is able to undergo mutarotation because it has a free anomeric hydroxyl group. (b) \( \alpha-\mathrm{D}(+) \) glucopyranose can also undergo mutarotation for the same reason, the free anomeric hydroxyl group. (c) \( \beta-\mathrm{D}(+) \) galactopyranose also has a free anomeric hydroxyl group, thus it can undergo mutarotation. (d) Methyl- \( \alpha \)-D-glucopyranoiside contains a methyl group bonded to the anomeric carbon, preventing the opening of the ring and thus cannot undergo mutarotation.

Step 3: Conclusion

Since mutarotation requires a free hydroxyl group at the anomeric carbon to allow the ring to open, compounds with an anomeric carbon bonded to a different atom rather than hydroxyl like a methyl group won’t undergo mutarotation. Hence, methyl-\( \alpha \)-D-glucopyranoiside will not undergo mutarotation.

Key Concepts

Anomeric Carbon

The anomeric carbon is a pivotal player in the phenomenon of mutarotation. It is specifically the carbon atom that becomes a new chiral center when a sugar forms a cyclic structure, such as in glucopyranose or galactopyranose. This transformation occurs when the hydroxyl group at the penultimate carbon attacks the carbonyl carbon, creating a hemiacetal or hemiketal.

• For glucopyranose and galactopyranose, this carbon is originally part of an aldehyde or ketone group in the open-chain form.
• The anomeric carbon is central to the ring closure that leads to two anomers, often denoted as alpha (α) and beta (β).
• A free hydroxyl group attached to this carbon makes the ring dynamic, allowing it to open and close, thereby facilitating the conversion between anomers.

Understanding the anomeric carbon is crucial because its bonding state determines whether a sugar can partake in mutarotation.

Optical Rotation

Optical rotation refers to the rotation of polarized light by a chiral substance in solution. When you dissolve an optically active compound, like a sugar, in solution, and pass polarized light through it, the orientation of the light is rotated. The extent of this rotation is dependent on:

• The concentration of the substance.
• The path length of the light through the solution.
• The specific rotation of the compound, a characteristic property of each chiral molecule.

Mutarotation directly affects optical rotation as it involves the conversion between different anomers, each with its distinctive optical activity. As the equilibrium shifts between these forms, the observed rotation can change until a stable equilibrium is reached. Understanding optical rotation is essential in studying the dynamic behavior of sugars in solution.

Glucopyranose

Glucopyranose is a term used for the cyclic form of glucose. In water, glucose predominantly exists in a six-membered ring known as pyranose. There are two anomers of glucopyranose, namely, alpha (α) and beta (β), which differ in the configuration around the anomeric carbon.

• The α-anomer has the hydroxyl group on the anomeric carbon trans (opposite side) to the CH₂OH group.
• The β-anomer positions the hydroxyl group cis (same side) to the CH₂OH group.

Due to the free hydroxyl group at the anomeric carbon in glucopyranose, the ring can open to revert to an open-chain form. It also facilitates conversion between α and β configurations, thereby showing mutarotation when in solution. Understanding the structure of glucopyranose helps in appreciating how such sugars behave dynamically in aqueous environments.

Galactopyranose

Galactopyranose is the cyclic form of galactose, a monosaccharide similar to glucose. It adopts a six-membered ring structure, or pyranose, in solution. Like glucopyranose, galactopyranose possesses two anomeric forms, α and β, influenced by the orientation of the hydroxyl group on the anomeric carbon.

• The α-galactopyranose has the hydroxyl group on the anomeric carbon positioned opposite to the CH₂OH group, while the β form has it on the same side.
• It plays a significant role in biological systems and is a component in glycolipids and glycoproteins.

Galactopyranose, due to its free anomeric hydroxyl group, can undergo mutarotation, transitioning between α and β forms when dissolved. This property is crucial for understanding its behavior in biological processes and in the context of mutarotation.