Problem 11 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following on reduction with \(\mathrm{NaBH}_{4}\) gives an equimolar mixture of sorbitol and mannitol? (a) mannose (b) glucose (c) xylose (d) fructose

Step-by-Step Solution

Answer

Fructose gives an equimolar mixture of sorbitol and mannitol on reduction with NaBH4.

Step 1: Analyze the Reduction Mechanism

When a carbonyl group is reduced with \( \mathrm{NaBH}_{4} \), aldehydes or ketones are reduced to alcohols. In the case of sugars, the carbonyl group at the C-1 position of aldoses or C-2 position of some ketoses is reduced. Glucose (an aldose) would result in a single product, sorbitol since it only has a C-1 ketone group.

Step 2: Understand Equimolar Mixtures

An equimolar mixture implies two products are formed in equal amounts. In the context of sugar reduction, this generally occurs when a ketose with a similar structure at the second carbon will produce two different stereoisomers as the carbonyl group is reduced.

Step 3: Examine Each Option

Mannose and glucose are aldoses, so they reduce to form a single alcohol each. Xylose is also an aldose. Fructose, however, is a ketose with a ketone group at the C-2 position, and its reduction can yield two different stereoisomers: sorbitol and mannitol.

Step 4: Conclusion from Analysis

Since fructose's C-2 ketone group allows for reduction to both sorbitol and mannitol, it is the only sugar in the options given that results in an equimolar mixture when reduced by \( \mathrm{NaBH}_{4} \).

Key Concepts

Reduction Reactions

Reduction reactions are a fundamental part of carbohydrate chemistry. They involve the addition of hydrogen or the removal of oxygen from a molecule. In the context of sugar chemistry, specific reagents like sodium borohydride ( \(\mathrm{NaBH}_{4} \)) are used to reduce carbonyl groups, which are C=O bonds found in aldehydes and ketones, to alcohols.
In the case of sugars, when an aldose (which has an aldehyde group) or a ketose (which has a ketone group) undergoes reduction, the carbonyl group is converted into an alcohol group. The outcome of such reactions is that the sugar structure maintains its carbon backbone, but now the oxygen's double bond is converted into a single bond with an OH group attached, thereby forming a polyol, or sugar alcohol. This is why reduction reactions are crucial for producing compounds like sorbitol and mannitol.
For example:

• In aldoses like glucose, the C-1 carbonyl group is targeted.
• In certain ketoses, like fructose, the C-2 carbonyl group is targeted during reduction.

Stereoisomers

Stereoisomers are molecules that have the same molecular formula and sequence of bonded atoms, but differ in the three-dimensional orientations of their atoms in space. In carbohydrate chemistry, this concept is critical because sugars often exist in multiple stereoisomeric forms. These small changes can dramatically impact a molecule's properties and biological activity.
When a ketose such as fructose is reduced to sorbitol and mannitol, two different stereoisomers are formed. The reduction of the carbonyl group creates a new chiral center. Depending on which face of the planar carbonyl group is attacked by the reducing agent, different arrangements result. This gives specific stereoisomers:

• Sorbitol and mannitol are stereoisomers of each other, with different orientations of hydroxyl groups.
• The formation of each stereoisomer is determined by the specific spatial arrangement during the reduction reaction.

This stereoisomerism is why fructose, when reduced, produces an equimolar mixture of both.

Ketoses vs. Aldoses

Understanding the differences between ketoses and aldoses helps in predicting the outcomes of reduction reactions. Aldoses have an aldehyde group at the end of their carbon chain, while ketoses have a ketone group typically on the second carbon atom.
These differences significantly impact the types of products formed during chemical reactions:

• Aldoses are generally more reactive to oxidation reactions because the aldehyde group can be easily oxidized.
• Ketoses, with their ketone group, often lead to more varied reduction products since the ketone can potentially form additional chiral centers during reduction.

In our specific example, fructose, which is a ketose, can reduce to yield two different sugar alcohols, contrasting with aldoses like glucose that typically yield one predictable product, such as sorbitol.

Sorbitol and Mannitol Production

Sorbitol and mannitol are sugar alcohols, and they derive from the reduction of sugars, which involves converting carbonyl groups to alcohol groups.
In the reduction of fructose, the ketone at the C-2 position is reduced, which can lead to the formation of both sorbitol and mannitol. This equimolar production occurs because the two possible outcomes arise from the formation of a new chiral center during reduction:

• Sorbitol: Retains a similar structure to its parent, glucose, as one of its stereoisomers.
• Mannitol: Is another outcome with a distinct chiral configuration compared to sorbitol.

These reactions are industrially exploited to produce sorbitol and mannitol, which are used commercially as sweeteners and in various pharmaceutical applications, thanks to their specific osmotic properties and lower caloric content than typical sugars.