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) says that all monosaccharides, whether aldose or ketose, act as reducing sugars. Typically, all monosaccharides are indeed reducing sugars because they have free aldehyde or ketone groups that can participate in redox reactions. Hence, statement (a) is correct.

Step 2: Analyze Statement (b)

Statement (b) suggests that bromine water can differentiate between aldoses and ketoses. Bromine water specifically oxidizes the aldehyde group in aldoses, thus reacting with them but not ketoses, which do not have free aldehyde groups. This property enables differentiation, making statement (b) correct.

Step 3: Analyze Statement (c)

Statement (c) claims that diastereomeric aldoses differing only at the C₂ configuration are anomers. Anomers differ in configuration at the carbonyl carbon (C₁ in aldoses); however, diastereomers differing only at C₂ are known as epimers. Thus, statement (c) is incorrect.

Step 4: Analyze Statement (d)

Statement (d) indicates that osazone formation destroys the configuration about C₂ of an aldose while preserving the rest. Osazone formation results in a reaction at C₂, forming a complex structure, and effectively changes the configuration at C₂ while the remaining configuration is unaffected. Therefore, statement (d) is correct.

Key Concepts

Reducing Sugars

Reducing sugars are a group of carbohydrates that have a free aldehyde or ketone group. This allows them to participate in oxidation-reduction (redox) reactions and "reduce" other chemicals. In the process, reducing sugars themselves get oxidized.
Examples of reducing sugars include:

• Glucose
• Fructose
• Lactose

All monosaccharides fall under this category, as they possess a free carbonyl group. This is essential for their role in biological redox processes, such as glycation in the human body or participating in the Maillard reaction during cooking.

Aldoses and Ketoses

Monosaccharides can be classified into aldoses and ketoses, depending on the presence of an aldehyde or ketone group.
**Aldoses** contain an aldehyde group at the first carbon, making it more reactive than other carbons. Examples of aldoses include glucose and galactose. Aldoses react with bromine water, oxidizing the aldehyde group.
**Ketoses**, on the other hand, have a ketone group typically at the second carbon. Fructose is a prime example. Ketoses do not react with bromine water in the same way, which helps distinguish them from aldoses.

Osazone Formation

Osazone formation is a reaction used to identify sugars, where sugars react with phenylhydrazine to form distinct crystalline structures known as osazones. This process is unique due to its effect on the molecule's configuration. The reaction converts the carbonyl group and the neighboring second carbon atom into a symmetrical structure.
Specifically:

• In aldoses, this affects the C₁ and C₂ configuration.
• The Osazone formation makes identifying the sugar type possible, yet destroys the configuration at C₂.

The rest of the sugar molecule remains intact, preserving its structural integrity and allowing for the deduction of sugar identity.

Diastereomers vs Anomers

In the world of carbohydrates, diastereomers and anomers represent different kinds of stereoisomers. It's crucial to understand the difference between them.
**Diastereomers** are stereoisomers that have multiple chiral centers and differ in configuration at one or more of these centers, without being mirror images of each other. A classic example is two sugars differing at the C₂ position but not being mirror images.
**Anomers**, in contrast, specifically differ at the anomeric carbon, which is the carbon derived from the carbonyl carbon on ring closure in sugars. For aldoses, this is typically at C₁. Anomers have distinct physical and chemical properties, which is why a small change, like the position of the -OH group at the anomeric carbon, can drastically alter a sugar's behavior.