Problem 128 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following give the same osazone on treatment with phenyl hydrazine? (a) Glucose (b) Galactose (c) Fructose (d) Mannose

Step-by-Step Solution

Answer

Glucose, mannose, and fructose give the same osazone.

Step 1: Understanding Osazone Formation

Osazone formation involves the reaction of sugars with phenylhydrazine to produce osazones. This reaction occurs on the carbon atoms involved in the glycosidic linkage and is useful in distinguishing sugars based on their structure.

Step 2: Identifying the Key Carbon Atoms

For aldohexoses, the osazone formation occurs primarily at the\( C_1 \) and \( C_2 \) carbon atoms. Glucose, mannose, and fructose have different structures but can form similar osazone due to interchangeable structural features at these positions.

Step 3: Comparing Chemical Structures

Glucose and mannose differ only at the configuration of the \( C_2 \) carbon atom, making their osazone formations identical. Fructose, a ketohexose, upon osazone formation, is identical to glucose and mannose as well. In contrast, galactose has a different configuration at the \( C_4 \) carbon, thus having a different osazone.

Step 4: Conclusion Based on Structures

Since glucose, mannose, and fructose yield osazones that are markedly similar, they give the same osazone upon treatment with phenylhydrazine. Galactose, due to distinct configuration changes, forms a different osazone.

Key Concepts

Phenylhydrazine Reaction

The phenylhydrazine reaction is a key process in carbohydrate chemistry, particularly useful for identifying and analyzing sugar structures. When sugars react with phenylhydrazine, they form compounds known as osazones. These reactions usually target the carbonyl group at specific positions in the sugar molecule. During this process, the sugar reacts with excess phenylhydrazine, where two phenylhydrazine molecules combine with the sugar, forming a unique osazone structure.
This reaction is crucial in determining similarities and differences in sugars like glucose, mannose, and fructose. It exploits structural features at the carbonyl group, mainly the first and second carbon atoms, accentuating subtle differences or similarities between them.

Carbohydrate Chemistry

Carbohydrate chemistry involves the study of sugar structures, properties, and reactions. Carbohydrates, which are organic compounds consisting of carbon, hydrogen, and oxygen, are categorized into monosaccharides, disaccharides, and polysaccharides. Monosaccharides are the simplest form of carbohydrates and include glucose, galactose, and fructose.

• **Monosaccharides**: Simple sugars like glucose and fructose. Essential for energy and cellular metabolism.
• **Disaccharides**: Formed by two monosaccharides, such as sucrose (glucose + fructose).
• **Polysaccharides**: More complex carbs, such as starch and cellulose, are made of multiple sugar units.

In carbohydrate chemistry, understanding the structure of sugar molecules is essential. It helps in determining how these sugars can react, as seen in the formation of osazones when reacting with phenylhydrazine. This reaction utilizes the inherent structural properties of sugars.

Aldohexoses vs Ketohexoses

Aldohexoses and ketohexoses are two subclasses of hexose sugars, differentiated by their functional groups. Aldohexoses, like glucose and mannose, possess an aldehyde group at the C1 position. Ketohexoses, like fructose, have a ketone group usually found at the C2 position.

• **Aldohexoses**: Six-carbon sugars with an aldehyde group. Common examples include glucose and mannose, which differ slightly in arrangement of their hydroxyl groups.
• **Ketohexoses**: These have a ketone functional group, with fructose being a prime example. Often, their structure allows shared osazone formation with some aldohexoses.

Although differing primarily in functional groups, the strategic position of reactive carbon atoms results in glucose, mannose, and fructose forming similar osazones when interacting with phenylhydrazine.

Sugar Structure Analysis

Analyzing sugar structures is vital for understanding how different sugars react and function biologically. Each sugar's structure dictates its physical and chemical properties. Structural analysis can expose similarities that might not be apparent through basic molecular formulas.
Through processes like osazone formation, one can reveal if sugars have interchangeable structures or identical configurations at crucial positions. For example,

• **Glucose**: Upon analysis, glucose forms identical osazones with mannose and fructose even though their basic structures differ due to similar arrangements at the C1 and C2 positions.
• **Mannose**: Shares a similar structure with glucose except for the configuration at the C2 carbon, yet forms the same osazone.
• **Fructose**: As a ketohexose, it reacts similarly due to conversion at the reactive sites during the osazone formation.

Studies in sugar structure analysis are essential for developing techniques to detect and differentiate sugars in various biological and chemical contexts.