Question

Mutarotation does not occur in (a) \(\mathrm{L}\) - glucose (b) D- glucose (c) sucrose (d) none of these

Short answer

Mutarotation does not occur in (c) sucrose.

Step-by-step solution

Understand Mutarotation

Mutarotation refers to the change in optical rotation that occurs when an alpha or beta anomer of a carbohydrate transforms into an equilibrium mixture of both. This process occurs in carbohydrates that can interconvert between different anomeric forms.

Analyze Sucrose

Sucrose is a disaccharide composed of glucose and fructose units linked through their anomeric carbons. Since both anomeric carbons are involved in the glycosidic linkage, no free anomeric carbon is available to interconvert to different forms. Hence, sucrose does not exhibit mutarotation.

Evaluate Other Options

Both \(\mathrm{L}\) - glucose and D-glucose are monosaccharides, and they possess free anomeric carbons. This enables them to exhibit mutarotation, unlike sucrose.

Conclusion

Since sucrose does not have a free anomeric carbon, it does not undergo mutarotation. On the other hand, the glucose molecules (L-glucose and D-glucose) can undergo mutarotation due to the presence of free anomeric carbons.

Key concepts

Anomeric Carbon

The anomeric carbon is a key player in carbohydrate chemistry. It is the carbon atom that was originally the carbonyl carbon (either aldehyde or ketone) in the open-chain form of the sugar. After the sugar cyclizes to form a ring, this carbon becomes a new stereocenter, or chiral center.
The configurational possibility at the anomeric carbon gives rise to two distinct stereoisomers called anomers. These are commonly referred to as the alpha (α) and beta (β) forms of the sugar. The designation α or β depends on the position of the hydroxyl group attached to the anomeric carbon relative to the rest of the sugar structure.
This carbon's ability to switch between forms is crucial for processes like mutarotation, where sugars can change from one anomer to another, often resulting in changes in optical rotation.

Optical Rotation

Optical rotation defines the way light is rotated as it passes through a solution of chiral molecules, like many carbohydrates. A substance that rotates the plane of polarized light is termed optically active.
The specific rotation of a compound is determined by:

• The nature of the compound.
• Its concentration in the solution.
• The length of the path the light travels through the solution.

In carbohydrates, like glucose, the existence of anomeric forms (α and β) means that each form has a unique optical rotation. When a solution of a carbohydrate is left to equilibrate, it often ends up as a mixture of both anomers, leading to an observed optical rotation that is a weighted average of the rotations of the pure anomers. This process is known as mutarotation.

Disaccharides vs Monosaccharides

Disaccharides and monosaccharides are both types of sugars, but they differ significantly. Monosaccharides are simple sugars with a basic molecular unit. Examples include glucose, fructose, and galactose. These sugars can exist in equilibrium between cyclic forms, allowing for mutarotation due to free anomeric carbons.
Disaccharides, on the other hand, are formed by two monosaccharide units joined together via a glycosidic bond. This linkage often involves the anomeric carbon of one or both sugar units, thereby restricting mutarotation. Sucrose is a prime example of a disaccharide where both anomeric carbons form the glycosidic bond, leaving no free anomeric carbon to undergo mutarotation.
Thus, while monosaccharides can freely interconvert between anomeric forms, disaccharides often lose this flexibility due to the binding of their anomeric carbons.