Question

This section contains multiple choice questions. Each question has 4 choices (a), (b), (c) and (d), out of which ONLY ONE is correct. Glucose on prolonged heating with HI and red phosphorus at \(373 \mathrm{~K}\) gives (a) sorbose (b) sorbitol (c) hexane (d) hexan-1-ol

Short answer

Answer: The product formed is hexane.

Step-by-step solution

Identify the properties of glucose

Glucose is a monosaccharide with the chemical formula \(C_6H_{12}O_6\). It has an aldehyde functional group at carbon 1 and five hydroxyl (-OH) groups on the remaining carbons.

Identify the properties of the reagents

The reagents used in this reaction are HI (hydroiodic acid) and red phosphorus. HI is a strong acid, and red phosphorus is often used as a reducing agent.

Understand the reaction mechanism

When glucose reacts with HI and red phosphorus at \(373 \mathrm{~K}\), all the hydroxyl groups and the aldehyde group in glucose are reduced. The reduction of the aldehyde group results in a primary alcohol group (hexan-1-ol or hexane), and the conversion of hydroxyl groups to hydrogen atoms may result in a primary alcohol or a hydrocarbon, either hexane or hexan-1-ol.

Analyze the given options and determine the correct answer

Based on the possible products formed and the given options, we can eliminate options (a) and (b), as neither sorbose or sorbitol are products of the given reaction. We are now left with options (c) and (d). Since the reaction includes a reduction of the aldehyde group and all of the hydroxyl groups, the correct answer is hexane (option c).

Key concepts

Reaction Mechanism

Understanding reaction mechanisms is fundamental in organic chemistry. A reaction mechanism is a step-by-step description of the process by which reactants are transformed into products. It describes how bonds are broken and formed in a chemical reaction, which is essential for predicting the reaction's outcome.
In the given exercise, glucose reacts with hydroiodic acid (HI) and red phosphorus. Here, HI acts as a reducing agent and red phosphorus assists the reaction process. This reaction illustrates a reduction mechanism, where glucose loses its functional groups.
The aldehyde group of glucose, along with its hydroxyl groups, undergoes reduction. An aldehyde group gets transformed into an alkane chain, resulting in a product like hexane. Thus, the understanding of the reaction mechanism is crucial to identify resulting products like hexane or other possible outcomes.

Monosaccharides

Monosaccharides are the simplest form of carbohydrates and are often referred to as simple sugars. Glucose is one of the most common and biologically important monosaccharides. It has the chemical formula \(C_6H_{12}O_6\) and serves as a primary energy source for cells.
Each carbon in a monosaccharide can carry a hydroxyl group, except for one, which may carry an aldehyde or ketone group. In glucose, this structure includes an aldehyde group, making it an aldose sugar.

• Monosaccharides are generally crystalline solids.
• They are soluble in water due to the presence of multiple hydroxyl groups.
• Monosaccharides like glucose play a crucial role in cellular respiration.

This basic understanding helps explain why glucose reacts the way it does under specific conditions with HI and red phosphorus in the exercise.

Reduction Reactions

Reduction reactions involve the gain of electrons or decrease in oxidation state, often converting a compound into a less oxidized form. In organic chemistry, reduction often results in the conversion of functional groups, such as converting aldehydes to alcohols or completely reducing compounds into alkanes.
In the given problem, glucose undergoes a reduction reaction. Both the aldehyde and hydroxyl groups in glucose are targeted by HI and red phosphorus, leading to complete reduction. This is why glucose eventually becomes hexane.

• Reduction of the aldehyde group in glucose results in an alcohol or further to an alkane.
• Reduction changes the structure significantly, removing oxygen atoms.
• Reduction is an important concept in organic chemistry due to its wide applications in synthetic chemistry.

The ability to trace these changes is integral to understanding how a compound's structure determines its chemical behavior and resulting products in reactions.