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

Option (d) is correct: condition-A gives aldonic acid and condition-B yields aldaric acid.

Step 1: Understanding the Reaction with Bromine Water

When an aldose is oxidized using \( \mathrm{Br}_2 / \mathrm{H}_2 \mathrm{O} \) (condition-A), the aldehyde group of the aldose is selectively oxidized to form an aldonic acid. The reaction does not affect the primary alcohol group present in an aldose, so the terminal hydroxyl group remains unchanged.

Step 2: Understanding the Reaction with Nitric Acid

When an aldose is oxidized using \( \mathrm{HNO}_3 \) (condition-B), both the aldehyde group and the primary alcohol group are oxidized. This reaction converts the aldose into an aldaric acid, where both ends of the molecule are oxidized to carboxylic acid groups.

Step 3: Comparing Product Formation in Conditions

In condition-A using \( \mathrm{Br}_2 / \mathrm{H}_2 \mathrm{O} \), we get aldonic acid, while in condition-B using \( \mathrm{HNO}_3 \), an aldaric acid is formed. This matches option (d) where condition-A gives aldonic acid and condition-B yields aldaric acid.

Key Concepts

Aldonic acid

Aldonic acids are formed when an aldose sugar undergoes a selective oxidation reaction. Specifically, the aldehyde group at the top of the aldose molecule is oxidized to form a carboxylic acid. Importantly, this reaction does not affect other parts of the molecule, such as the primary alcohol at the end of the sugar.

• The oxidation typically uses a mild oxidizing agent, like bromine water (\(\mathrm{Br}_2/\mathrm{H}_2\mathrm{O}\)).
• After the reaction, the sugar now has a carboxyl group (-COOH) instead of a terminal aldehyde (-CHO).

This transformation is crucial because it selectively modifies the sugar, allowing for further chemical studies and applications. Aldonic acids, due to this oxidation, differ in chemical properties from their parent aldoses, often increasing their solubility in water.

Aldaric acid

Aldaric acids are produced when both the aldehyde and the primary alcohol groups in an aldose are oxidized. This complete oxidation is typically achieved with a more potent oxidizing agent, such as nitric acid (\(\mathrm{HNO}_3\)). Both ends of the sugar molecule are converted into carboxylic acid groups.

• Each aldose gives rise to a specific aldaric acid, with unique properties.
• This modification can lead to changes in the sugar's physical and chemical behavior.

Aldaric acids are less common in biological systems compared to aldonic acids but are vital in various industrial and laboratory settings due to their dual oxidation, which yields symmetric molecules.

Bromine water oxidation

Bromine water oxidation specifically targets the aldehyde group on an aldose, converting it into an aldonic acid. This method is favored for its selectivity and mildness, which means it does not tamper with other functional groups within the sugar.

• In the reaction, the bromine acts as an oxidant, and the presence of water facilitates the conversion.
• The simplicity of this reaction makes it popular in laboratory settings for preparing aldonic acids easily.

A key aspect of bromine water oxidation is its ability to yield products that maintain the integrity of the sugar's carbon backbone, leading to chemicals that are structurally similar to the starting material but with an oxidized aldehyde group.

Nitric acid oxidation

Nitric acid oxidation is a powerful process used to transform aldose sugars into aldaric acids by oxidizing both the aldehyde and primary alcohol groups. Due to its stronger oxidative power, nitric acid can efficiently modify both ends of the sugar molecule.

• This oxidation offers chemists a way to fully open and oxidize a sugar's structure, enabling the creation of symmetric molecules.
• While this reaction is more aggressive compared to others, it is controlled to selectively oxidize the desired groups without damaging the carbon chain.

Using nitric acid for oxidation serves not just laboratory uses but also in industrial applications where comprehensive sugar modification is needed. The resulting aldaric acids often provide more reactive frameworks for further chemical synthesis.