Problem 131 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

Statements (a), (b), and (d) are correct.

Step 1: Analyze Statement (a)

Statement (a) claims that all monosaccharides act as reducing sugars. Monosaccharides such as aldoses can typically act as reducing sugars because they have a free aldehyde group. Ketoses can also act as reducing sugars, but only when they tautomerize to aldoses under basic conditions. Therefore, the statement is generally correct, as both aldoses and ketoses can be reducing sugars under the right conditions.

Step 2: Analyze Statement (b)

Statement (b) claims that bromine water can be used to differentiate aldoses from ketoses. Bromine water oxidizes the aldehyde group in aldoses but does not react with ketoses under mild conditions. Therefore, it can indeed be used to differentiate between these two classes of sugars, making this statement correct.

Step 3: Analyze Statement (c)

Statement (c) defines a pair of diastereomeric aldoses differing only at \(C_{2}\) as anomers. Anomers differ in configuration at the anomeric carbon, which is the carbonyl carbon (usually \(C_{1}\) in aldoses), not \(C_{2}\). Thus, the statement is incorrect.

Step 4: Analyze Statement (d)

Statement (d) concerns osazone formation and its effect on stereochemistry. Osazone formation involves the reaction of phenylhydrazine with the carbonyl carbon and the adjacent carbon (usually \(C_{2}\) in aldoses), which indeed alters the configuration at \(C_{2}\) but does not affect other parts of the molecule. Therefore, this statement is correct.

Key Concepts

Monosaccharides

Monosaccharides are the simplest form of carbohydrates and are often referred to as the building blocks of more complex carbohydrates like starches and cellulose. These molecules typically consist of a single sugar unit with a general formula of \((CH_2O)_n\), where \(_n_\) ranges typically from 3 to 7. This structure contains multiple hydroxyl groups (-OH) and either an aldehyde or a ketone group. Monosaccharides are classified based on the number of carbon atoms present and whether they contain an aldehyde group (aldoses) or a ketone group (ketoses). Here are some key features:

• They are soluble in water and play essential roles in energy metabolism.
• Common examples include glucose, fructose, and galactose.
• Monosaccharides can participate in various biochemical reactions, highlighting their versatility in biological systems.

While simple in structure, these sugars are crucial for various physiological functions, including acting as reducing agents.

Reducing Sugars

Reducing sugars are those that can donate electrons to another chemical species, meaning they have the capability to reduce other compounds. This property is due to the presence of a free aldehyde or ketone group in their structure. In a reducing sugar:

• Monosaccharides such as glucose and galactose are prime examples because they contain a free aldehyde group.
• Ketoses, like fructose, can also be reducing sugars, but they first undergo tautomerization to form aldoses.
• The reducing nature of sugars like glucose is commonly tested using reagents like Benedict's solution, where a color change indicates the reduction process.

This chemical property is not only used in lab tests to identify sugar types but also plays significant roles in metabolic pathways, where the transfer of electrons is a fundamental process.

Aldoses and Ketoses

Aldoses and ketoses are two categories of monosaccharides based on the functional group present. Aldoses contain an aldehyde group at the end of the molecule, whereas ketoses have a ketone group, which is usually located at the second carbon atom from the end. The differences between these two can be summarized as:

• Aldoses, such as glucose, are capable of being oxidized and can easily participate in chemical reactions.
• Ketoses, like fructose, are less reactive but can tautomerize into aldoses under basic conditions, allowing them to partake in reactions like reducing sugars.
• The ability to undergo reactions varies, with aldoses readily oxidized by agents such as bromine water, whereas ketoses do not react without additional conditions.

This classification is important for understanding the reactivity and properties of carbohydrates in both artificial and natural environments.

Osazone Formation

The reaction of osazone formation is an interesting chemical test used to differentiate reducing sugars. It involves the reaction of the sugar with excess phenylhydrazine. During this process, the following occurs:

• The reaction targets the carbonyl group and the adjacent carbon, typically impacting the C1 and C2 carbons in aldoses.
• This reaction is characterized by the formation of crystalline derivatives known as osazones.
• Osazone formation significantly alters the configuration of the C2 carbon, which is essential in distinguishing between different sugars.

These derivatives have distinct crystalline shapes, which can be used as a visual tool to identify specific sugars. This chemical technique has historical importance in biochemistry for characterizing monosaccharides in complex mixtures.