Problem 127 - A Complete Resource Book in Chemistry for JEE Main

Question

An aldose is oxidized separately by \(\mathrm{Br}_{2} / \mathrm{H}_{2} \mathrm{O}\) (condition-A) and by \(\mathrm{HNO}_{3}\) (condition-B). Which of the following is the correct combination of the products formed? (a) condition-A gives aldaric acid while condition-B yields a mixture of aldonic and aldaric acids (b) condition-A gives aldonic acid while condition-B yields a mixture of aldonic and aldaric acids (c) condition-A gives aldaric acid while condition-B yields aldonic acid (d) condition-A gives aldonic acid while condition-B yields aldaric acid

Step-by-Step Solution

Answer

Condition-A gives aldonic acid; Condition-B gives aldaric acid. Correct answer: (d).

Step 1: Understanding Oxidation by Br₂/H₂O (Condition-A)

When an aldose is treated with bromine water (\( \mathrm{Br}_{2} / \mathrm{H}_{2}O \)), the aldehyde group at the end of the aldose is selectively oxidized to a carboxylic acid without affecting the primary alcohol group at the other end of the sugar. This reaction converts the aldose into an aldonic acid.

Step 2: Understanding Oxidation by HNO₃ (Condition-B)

When an aldose is treated with nitric acid (\( \mathrm{HNO}_{3} \)), both the aldehyde group and the primary alcohol group are oxidized to carboxylic acids. This reaction converts the aldose into an aldaric acid as both ends of the carbohydrate chain are oxidized.

Step 3: Identifying the Correct Combination

By understanding how each oxidizing agent reacts with an aldose, we can identify the products formed. Condition-A produces aldonic acid (only oxidizes the aldehyde to a carboxylic acid), and condition-B produces aldaric acid (oxidizes both the aldehyde and primary alcohol to carboxylic acids). This matches with option (d).

Key Concepts

Aldonic Acid

Aldonic acids play a crucial role in carbohydrate chemistry. They are formed through the oxidation of an aldose sugar.
When chemists use bromine water (\( \mathrm{Br}_{2} / \mathrm{H}_{2}O \)) to react with an aldose, they intentionally target the aldehyde group present at one end of the sugar molecule. The reaction results in converting this aldehyde group into a carboxylic acid.

• The primary alcohol group on the other end is left unchanged, distinguishing aldonic acids from other oxidized forms.
• The reaction occurs under mild conditions, which is why it's selective for the aldehyde group.
• Examples of aldonic acids include gluconic acid derived from glucose, and mannonic acid from mannose.

Understanding aldonic acids is fundamental when dissecting or modifying sugar compounds for specific chemical processes or synthesizing biochemically relevant molecules.

Aldaric Acid

Aldaric acids are another important class of oxidized sugars. The critical difference from aldonic acids is the level of oxidation. When an aldose sugar undergoes oxidation with nitric acid (\( \mathrm{HNO}_{3} \)), both ends of the sugar molecule are oxidized.

• This transformation converts both the aldehyde group and the primary alcohol group into carboxylic acids.
• This reaction is more aggressive compared to oxidation with bromine water.
• For example, glucose when fully oxidized by \( \mathrm{HNO}_{3} \) becomes glucaric acid.

Understanding aldaric acids is essential for applications in material science and pharmaceutical research, offering insights into developing advanced biopolymers and drug intermediates.

Oxidizing Agents

Oxidizing agents are pivotal in chemical reactions, especially in the field of organic chemistry, where they are used to modify functional groups. Among these agents, bromine water and nitric acid are commonly used oxidants in the context of sugar chemistry.

• Bromine water is a milder oxidizing agent. It selectively oxidizes the aldehyde group in aldoses, leading to the formation of aldonic acids.
• It is effective due to its gentle action, preventing over-oxidation of the alcohol group into carboxylic acids.


• In contrast, nitric acid is a stronger oxidizing agent. It causes oxidation at both ends of the sugar molecule, turning both the aldehyde and the primary alcohol groups into carboxylic acids. This intense reaction leads to the formation of aldaric acids.
• Choosing the right oxidizing agent depends on the desired chemical outcome, highlighting their critical role in synthetic transformations.

Understanding these agents and their specific actions can help in designing efficient and precise chemical syntheses.