Problem 146 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Which of the following pairs of sugars form identical osazone derivative? (a) \(\mathrm{D}\) - \((+)\) -Glucose and \(\mathrm{D}-(-)\) -arabinose (b) \(\mathrm{D}\) - \((+)\) -Glucose and \(\mathrm{D}\) - \((-)\) -fructose (c) D-(+)-Glucose and (+)-maltose (d) \(\mathrm{D}-(+)\) -Glucose and \((+)\) -lactose

Step-by-Step Solution

Answer

(b) D-(+)-Glucose and D-(-)-Fructose form identical osazone derivatives.

Step 1: Identify Functional Groups

To solve this problem, we need to know that an osazone is formed by the reaction of the sugar with phenylhydrazine. In the reaction, only the carbon atoms involved in the formation of osazones are the carbonyl group (C=O) and the adjacent carbon atom (C-2). In this step, identify that for both aldoses and ketoses, the C-1 and C-2 positions are reactive.

Step 2: Understand the Reaction Influences

In the formation of osazones, both the ketose and the corresponding aldose react with phenylhydrazine, where the C-1 and C-2 positions are involved. The C-2 epimers give the same osazone derivative because the rest of the molecule remains identical.

Step 3: Review Glucose and Fructose Structure

D-(+)-Glucose is an aldose, and D-(-)-Fructose is a ketose. Both sugars share the same configuration at all carbons except the C-1 position of glucose (aldehyde) and the C-2 position of fructose (ketone). Since osazone formation involves these two positions, glucose and fructose form identical osazones.

Step 4: Evaluate Other Pairs

Although D-(+)-Glucose and D-(-)-Arabinose share similar configurations at some carbons, the presence of significant differences beyond C-2 beyond makes them form different osazones. Similarly, the pairings with maltose and lactose involve disaccharides, which when hydrolyzed do not match glucose's configurations at C-1 and C-2 required for identical osazone formation.

Step 5: Conclusion of Analysis

The pair that forms identical osazone derivatives due to having the same structural pattern at C-1 (glucose, as an aldehyde) and C-2 (fructose, as a ketone) is D-(+)-Glucose and D-(-)-Fructose.

Key Concepts

Aldoses and Ketoses

Aldoses and ketoses are two types of sugars that differ primarily in the location of their carbonyl group. This difference is important in carbohydrate chemistry for understanding how these sugars react with other compounds. An aldose contains an aldehyde group at the first carbon atom (C-1), while a ketose features a ketone group located at the second carbon atom (C-2).
A simple way to remember the distinction:

• Aldose = aldehyde at the top/first position.
• Ketose = ketone at the second position.

In reactions, such as osazone formation, both aldoses and ketoses show reactivity due to these functional groups.
This difference can help us figure out how each sugar behaves in chemical reactions, including the formation of identical derivatives. Understanding these differences is crucial for deducing the chemical properties and reactions of carbohydrates, framing the context for the phenylhydrazine reaction.

Phenylhydrazine Reaction

The phenylhydrazine reaction is a specific chemical reaction used to identify and study sugars like aldoses and ketoses. This reaction involves a sugar and phenylhydrazine, forming a compound known as an osazone.
Only the C-1 and C-2 positions in the sugar molecule are involved in this process, leading to the formation of the osazone. Here is how it works:

• The carbonyl group constitutes the main reactive site.
• In aldoses, it reacts at the C-1 position (aldehyde group), whereas in ketoses, it acts at the C-2 position (ketone group).

As the reaction progresses, the phenylhydrazine reacts with the sugar to eliminate oxygen and add nitrogen times three, thus forming the osazone.
The phenylhydrazine reaction is unique due to its ability to produce identical osazone structures from some pairs of sugars, illustrating the biochemical similarity in their structure beyond the first two carbon positions.

Sugar Chemistry

Sugar chemistry is the study of the structure and function of sugar molecules, which are essential elements of carbohydrates. Understanding sugar chemistry is crucial for processes such as energy production and cellular communication.
In terms of their chemical composition, sugars like glucose and fructose are made up of carbon (C), hydrogen (H), and oxygen (O) atoms.

• Monosaccharides, like glucose and fructose, are the simplest carbohydrates and serve as building blocks for more complex molecules.
• Disaccharides, such as maltose and lactose, are formed from two monosaccharide units.

In processes like osazone formation, the structural nuances, such as the arrangement of atoms around the carbon atoms, are compared, which results in understanding the similarities and differences among different sugars.
Within the realm of sugar chemistry, the formation of derivatives, such as osazones, is vital for the detailed study of sugars and their behaviors in various environments and reactions.

Carbohydrate Isomerism

Carbohydrate isomerism explores the existence of compounds with the same molecular formula but different structural forms, which is a pivotal concept in sugar chemistry.
This phenomenon is found in various forms, such as:

• Stereoisomers, which includes enantiomers and diastereomers, showcase the same structural framework but differ in the spatial arrangement of atoms.
• Epimers are a type of diastereomer that differ at only one chiral center.

When we examine carbohydrates, the subtle differences in the arrangement of functional groups lead to distinct isomeric forms.
For example, glucose and fructose are an aldose-ketose pair, where their structures differ slightly. Yet during the phenylhydrazine reaction, this pair forms identical osazones due to the reactions at their specific reactive centers (C-1 in glucose and C-2 in fructose). The study of isomerism in carbohydrates is essential for understanding the diversity and reactivity of sugar molecules in biological systems and their applications in biochemical research.