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 is the correct answer.

Step 1: Understand the Problem

The problem asks us to determine which of the listed sugars, when reduced with sodium borohydride (\(\mathrm{NaBH}_{4}\)), forms an equimolar mixture of sorbitol and mannitol.

Step 2: Identify Product Formation from Reduction

Sodium borohydride (\(\mathrm{NaBH}_{4}\)) is a reducing agent that typically reduces aldehydes and ketones to the corresponding alcohol. It is important to know which structure of the given options can yield an equimolar mixture of sorbitol and mannitol upon reduction.

Step 3: Analyze Sugar Structures

Review the structures of the given sugars: - Mannose and glucose have an aldehyde group, which reduces to a single type of alcohol. - Xylose has an aldehyde group with fewer carbon atoms, yielding a different alcohol. - Fructose is a ketohexose, having a ketone group at the second position.

Step 4: Determine Possible Reaction for Each Option

- Mannose reduces to sorbitol only. - Glucose reduces to sorbitol only. - Xylose reduces to a sugar alcohol unrelated to sorbitol or mannitol. - Fructose, due to its keto group, can yield two products (sorbitol and mannitol) upon reduction as the keto group can reduce in two ways, each leading to one of the two possible products.

Step 5: Identify Correct Option

Based on the analysis above, fructose is the only option that, upon reduction with \(\mathrm{NaBH}_{4}\), yields an equimolar mixture of both sorbitol and mannitol because of its symmetrical keto group reduction.

Key Concepts

Sorbitol

Sorbitol is an important sugar alcohol derived from the reduction of certain sugars. It is not only utilized in scientific studies but is also widely used in food and cosmetic industries. The reduction of glucose using sodium borohydride (\(\mathrm{NaBH}_{4}\)) typically results in sorbitol.

• Sorbitol is classified as a polyol, a type of carbohydrate with a sweet taste.
• Unlike regular sugar, sorbitol has fewer calories, making it a popular sugar substitute.
• It is commonly found in "sugar-free" foods and oral hygiene products like toothpaste.

When glucose undergoes a reduction reaction with \(\mathrm{NaBH}_{4}\), the aldehyde group in glucose is transformed into an alcohol group, resulting in sorbitol. This process illustrates the simplicity and efficacy of hydrogenation reactions in transforming simple sugars into useful derivatives.

Mannitol

Mannitol is another sugar alcohol similar to sorbitol and results from the reduction of certain sugars. It is often produced alongside sorbitol, especially when specific sugars, like fructose, undergo reduction.

• Mannitol is recognized for its application in medical settings, especially as a diuretic, which promotes the expulsion of excess fluids from the body.
• It is also notable for its use in managing intracranial pressure.
• Like sorbitol, mannitol serves as a sugar substitute in various food products.

By using \(\mathrm{NaBH}_{4}\), fructose is reduced to yield an equimolar mixture of mannitol and sorbitol. This results from the unique structure of fructose, which allows its ketone group to be reduced in two different orientations, forming both sugar alcohols.

Fructose Reduction

Fructose reduction with sodium borohydride (\(\mathrm{NaBH}_{4}\)) is an interesting example that produces two distinct sugar alcohols: sorbitol and mannitol. The structure of fructose plays a central role in this process.

• Fructose is known as a ketohexose due to its six-carbon skeleton and a ketone group at the second carbon.
• This ketone group can be reduced in two different ways, leading to the production of both sorbitol and mannitol.
• The equimolar production of these two alcohols results from the ketone group being located symmetrically on fructose, allowing for two equivalent reduction pathways.

By utilizing \(\mathrm{NaBH}_{4}\), a favorable and efficient chemical route is established, transforming fructose into valuable low-calorie sugar alcohols beneficial in numerous applications, including food and pharmaceuticals. This dual reduction outcome highlights the versatility of fructose in biochemical and industrial processes.