Problem 79 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

\(\mathrm{D}-(+)\) -Glucose shows mutarotation because (a) it undergoes interconversion with \(\mathrm{D}-(-)\) -fructose (b) it undergoes interconversion between its pyranose structure and furanose structure (c) it undergoes interconversion between its \(\alpha\) -D-(+)-glucopyranose and \(\beta-\mathrm{D}-(+)\) glucopyranose structures via the open-chain structure (d) it is dextrorotatory

Step-by-Step Solution

Answer

Mutarotation in glucose occurs due to interconversion between b1 and b2 forms via the open-chain structure, so the answer is (c).

Step 1: Understand Mutarotation

Mutarotation refers to the change in the optical rotation due to the change in the equilibrium between two anomers, b1 and b2, of a sugar.

Step 2: Analyze Glucose Structures

Glucose exists in different structural forms, including the open-chain form and the cyclic pyranose forms (b1 and b2 anomers). The change in rotation occurs as the sugar goes from one anomer to another.

Step 3: Determine Conditions for Interconversion

The interconversion between the b1 and b2 glucopyranose forms requires the open-chain structure as an intermediate in the reaction under normal conditions in aqueous solutions.

Step 4: Identify Correct Option

In glucose, mutarotation is caused by the interconversion between b1-D-(+)-glucopyranose and b2-D-(+)-glucopyranose via the open-chain form. Therefore, the correct answer is option (c).

Key Concepts

Glucose Anomers

Anomers are types of isomers specific to carbohydrates like glucose. These differ in the configuration around the carbon atom known as the anomeric carbon, which is the first carbon atom in the sugar ring. In the case of glucose, this refers to the \(C_1\) atom. When glucose forms a ring structure, it can form two different anomers known as \(\alpha\) and \(\beta\).

These two forms are called \(\alpha-D-(+)\)-glucopyranose and \(\beta-D-(+)\)-glucopyranose. The difference between these two anomers is how the hydroxyl group attached to the anomeric carbon is oriented:

• In the \(\alpha\) anomer, the hydroxyl group is on the opposite side (trans) of the ring as the \(-CH_2OH\) group.
• In the \(\beta\) anomer, it is on the same side (cis) as the \(-CH_2OH\) group.

Mutarotation in glucose involves the equilibrium of these two anomers through an intermediate open-chain form. During this, the optical rotation of the solution changes, fitting perfectly into the concept of mutarotation.

Optical Rotation in Sugars

Sugars like glucose are optically active, which means they can rotate plane-polarized light. The measurement of how much they rotate this light is called optical rotation. When glucose dissolves in water, it exhibits an interesting phenomenon where its optical rotation changes over time. This change is due to mutarotation.

When \(\alpha-D-(+)\)-glucopyranose first dissolves, its optical rotation might be different than when \(\beta-D-(+)\)-glucopyranose dissolves due to their structural differences. Over time, in solution, these anomers interconvert until they reach equilibrium. This interconversion adjusts the optical rotation until a constant value is reached.

• Initial Optical Rotation: This could differ based on starting form - \(\alpha\) might be more or less than \(\beta\).
• Equilibrium Optical Rotation: This remains constant once equilibrium is met between \(\alpha\) and \(\beta\) forms.

This ability to change light direction is crucial in determining the nature of substances in chemistry and understanding carbohydrate behaviors like mutarotation.

Pyranose and Furanose Structures

Pyranose and furanose refer to different cyclic forms of sugars. These rings form when the aldehyde or ketone group on a sugar reacts with an alcohol group elsewhere in the molecule, closing the chain.

• Pyranose structure: This form is a six-membered ring resembling the structure of pyran. An example is when glucose forms \(\alpha\)-D-(+) or \(\beta\)-D-(+) glucopyranose.
• Furanose structure: A five-membered ring similar to furan. It forms less frequently in glucose, but is commonly found in other sugars like fructose.

In the case of glucose, the pyranose form is more stable, which is why \((\alpha-\) and \(\beta-\) glucopyranose are the predominant forms. The stability comes from minimizing torsional strain and maximizing its internal hydrogen bonds.

Understanding these structures is essential because they help predict how sugars behave chemically and biologically. Additionally, pyranose and furanose ring formations allow sugars to carry out various biological functions efficiently.