Problem 125 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following statements about carbohydrates: 1\. Bromine water can be used to differentiate an aldose from a ketose 2\. All monosaccharides, whether aldose or ketose, are reducing sugars 3\. Osazone formation destroys the configuration about \(\mathrm{C}-2\) of an aldose, but does not affect the configuration of the rest of the molecule 4\. A pair of diastereomeric aldoses which differ only in configuration about \(\mathrm{C}-2\) is termed as pair of anomers Which of the above statements are correct? (a) 1 and 4 (b) 2 and 4 (c) 1,2 and 3 (d) 2,3 and 4

Step-by-Step Solution

Answer

The correct statements are 1, 2, and 3: option (c).

Step 1: Evaluate Statement 1

Statement 1 claims that bromine water can differentiate an aldose from a ketose. Bromine water is known to oxidize aldehydes to carboxylic acids without affecting ketones. This means it can differentiate between aldose (which has an aldehyde group) and ketose (which has a ketone group). Hence, Statement 1 is correct.

Step 2: Evaluate Statement 2

Statement 2 states all monosaccharides, whether aldose or ketose, are reducing sugars. Reducing sugars can donate electrons to other molecules. Aldoses, due to their aldehyde group, are reducing sugars. Ketoses can also be reducing sugars because they can tautomerize to form an aldose structure in solution, making Statement 2 correct.

Step 3: Evaluate Statement 3

Statement 3 states that osazone formation destroys the configuration about \(\mathrm{C}-2\) of an aldose, but not the rest of the molecule. Osazone formation involves reaction with phenylhydrazine that can disrupt the configuration around \(\mathrm{C}-2\) but keeps the rest of the structure intact, proving Statement 3 correct.

Step 4: Evaluate Statement 4

Statement 4 claims that diastereomeric aldoses differing only in configuration around \(\mathrm{C}-2\) are called anomers. However, anomers specifically differ in configuration only at the anomeric (formerly carbonyl) carbon, \(\mathrm{C}-1\). Thus, Statement 4 is incorrect because the difference between diastereomers at \(\mathrm{C}-2\) is called epimers, not anomers.

Step 5: Choose the Correct Option

Given the evaluations, Statements 1, 2, and 3 are correct. Therefore, the correct choice corresponds to option (c): 1, 2, and 3.

Key Concepts

Aldose

Carbohydrates are essential biomolecules, and among them, aldoses stand out due to their unique structural feature - an aldehyde group. In simple terms, aldoses are sugars that contain an aldehyde group (-CHO) at the end of their carbon chain. This is important because the aldehyde group makes them reactive.

Some examples of aldoses include glucose and galactose. They are commonly found in many fruits and play a vital role in energy metabolism within the body. Aldoses can participate in certain chemical reactions where their aldehyde group is oxidized, which makes them useful in differentiating them from other sugars.

• Aldoses have a reactive aldehyde group.
• They are found naturally, like in glucose.
• Important for body's energy needs.

A common laboratory technique to recognize them involves using bromine water, which can oxidize the aldehyde group to an acid, helping to distinguish them from their cousin, ketoses.

Ketose

Ketoses are carbohydrates characterized by having a ketone group ( > =C=O located typically at the second carbon in the chain. One of the most well-known ketoses is fructose, often found in fruit and honey. Unlike aldoses, ketoses have a slightly different reactivity due to their ketone group.

In aqueous solutions, ketoses can undergo a transformation, known as tautomerization, where they convert into an aldose form. This ability allows ketoses to exhibit similar properties to aldoses in certain reactions.

• Contain a ketone group instead of an aldehyde.
• Fructose is a common ketose.
• Can convert into aldoses in solution.

Understanding ketoses is crucial because while they also function as reducing sugars, their conversion ability broadens their reactivity and application in biochemical processes.

Osazone Formation

Osazone formation is a fascinating reaction in carbohydrate chemistry. When an aldose reacts with phenylhydrazine, the process transforms the sugar into a yellow crystalline compound called osazone. This is important for identification purposes, as different sugars create osazones with slightly varying shapes, aiding in distinguishing them in laboratory settings.

In the case of aldoses, this reaction impacts the configuration around the > C-2 carbon, meaning the original structure at this position is altered. However, the rest of the molecule remains unchanged.

• Reaction with phenylhydrazine.
• Forms a unique crystal structure.
• Helpful for distinguishing particular sugars.

Osazone formation is particularly useful for studying sugars because it provides visual evidence via crystal formation, presenting an additional tool for scientists to analyze and categorize carbohydrates.

Reducing Sugars

Reducing sugars are intriguing due to their chemical capability to act as electron donors. This means they can reduce other chemicals during reactions. The term "reducing" stems from their ability to reduce copper(II) ions (from solutions like Benedict’s or Fehling’s) to copper(I) ions, forming a red precipitate of cuprous oxide.

Both aldoses and some ketoses are reducing sugars. Aldoses naturally reduce substances due to their free aldehyde group. Ketoses, although initially different, can sometimes tautomerize or convert into an aldose form in solution, adopting the same reducing properties.

• Act as electron donors in reactions.
• Can reduce copper(II) ions to copper(I).
• Aldoses naturally have reducing ability.
• Ketoses may convert to aldoses to exhibit reducing properties.

Understanding what makes a sugar a reducing one is important in both food industry applications, like testing for sugar content, and in medical diagnostics, such as detecting glucose levels in diabetes management.