Problem 125 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following statements about carbohydrates: 1\. Bromine water can be used to differentiate an aldose from a ketose 2\. All monosaccharides, whether aldose or ketose, are reducing sugars 3\. Osazone formation destroys the configuration about C-2 of an aldose, but does not affect the configuration of the rest of the molecule 4\. A pair of diastereomeric aldoses which differ only in configuration about C-2 is termed as pair of anomers Which of the above statements are correct? (a) 1 and 4 (b) 2 and 4 (c) 1,2 and 3 (d) 2,3 and 4

Step-by-Step Solution

Answer

Correct statements are 1, 2, and 3; answer (c).

Step 1: Understanding Aldoses and Ketoses

Aldoses have an aldehyde group, while ketoses have a ketone group at C-2. Bromine water can be used to oxidize aldoses to aldonic acids, which is a way to differentiate them from ketoses.

Step 2: Reducing Sugars Identification

All monosaccharides, including both aldoses and ketoses, are reducing sugars. Ketoses convert to aldoses in basic solutions before acting as reducing agents, thus both are reducing sugars.

Step 3: Osazone Formation

The process of forming osazone involves phenylhydrazine reacting with the carbonyl group of carbohydrates. This reaction alters the configuration at C-2 for both aldoses and ketoses, but does not affect the configuration of the rest of the molecule.

Step 4: Understanding Anomers and Diastereomers

Anomers are isomers that differ at a new asymmetric carbon formed on ring closure. Diastereomers differ at one or more but not all chiral centers. Statement 4 relates to diastereomers, not anomers, as changing C-2 configuration creates an epimer.

Step 5: Evaluating Statements and Selecting Answer

Statement 1 is true as bromine water identifies aldoses. Statement 2 is correct because all individual monosaccharides are reducing sugars. Statement 3 is also correct about osazone formation at C-2. Statement 4 is incorrect as it describes epimers, not anomers, as differences at C-2.

Key Concepts

Aldoses and Ketoses

When discussing carbohydrates, it's important to understand two main classes: aldoses and ketoses. These terms refer to the type of carbonyl group present in the sugar.

• Aldoses: These sugars have an aldehyde group at the end of the carbon chain. The simplest form is glyceraldehyde.
• Ketoses: These sugars feature a ketone group, typically at the second carbon (C-2). An example is dihydroxyacetone.

A practical laboratory method to differentiate between these is using bromine water. Bromine water can oxidize aldoses to form aldonic acids, showcasing a clear distinction because ketoses are not oxidized under these conditions.
This chemical behavior stems from the reactive nature of the aldehyde group, which is absent in ketoses, making it a handy test tool for identifying aldoses.

Reducing Sugars

Reducing sugars are carbohydrates that can reduce other chemicals, a property stemming from their free anomeric carbons. All monosaccharides, which include both aldoses and ketoses, qualify as reducing sugars.
For aldoses, the reducing nature is directly due to the aldehyde group.
Ketoses, though initially containing a ketone group, can transform under alkaline conditions to form an isomeric aldose.

• Significance: This transformation allows ketoses to behave as reducing sugars, reacting with agents like Fehling's solution or Benedict's reagent.

Understanding this property is crucial in biochemical tests for sugars, as reducing sugars play vital roles in metabolic pathways and energy production.

Osazone Formation

Osazone formation is a historic method to study sugars by visualizing them as crystals. When phenylhydrazine reacts with monosaccharides, an osazone is formed, giving these sugars unique crystal structures.

• The reaction specifically targets the carbonyl group, affecting the configuration at the C-2 position.
• Noticeably, it doesn't alter other parts of the sugar molecule.

In practical terms, though, it means that different sugars can form identical osazones if they only differ at the first two carbon atoms, which is common across various aldoses and ketoses.
This nuanced chemical reaction provided insights into early carbohydrate research even though it fell out of regular use due to more modern techniques.

Diastereomers and Anomers

Diastereomers and anomers are specific types of stereoisomers found in carbohydrates. They provide insight into the three-dimensional arrangement of sugar molecules.

• Diastereomers: These are sugars that differ at one or more (but not all) chiral centers. Such differences can profoundly affect properties like taste and solubility.
• Anomers: A subtype of diastereomers, these differ at the new chiral center formed when a sugar molecule forms a cyclic structure from its linear form. This new chiral center is typically found at the anomeric carbon.

It's key to note that when sugars cyclize and form a ring, they can form two anomeric forms: alpha (b1) and beta (b2).
One common misconception is treating diastereomeric differences at C-2 as anomeric differences, which is incorrect since anomers specifically concern the cyclic structure's chiral centers.