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 \(\mathrm{C}-2\) 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

Short answer

The correct statements are 1, 2, and 3. The correct answer is (c).

Step-by-step solution

Analyzing Statement 1

Statement 1 claims that bromine water can differentiate an aldose from a ketose. Bromine water is an oxidizing agent that oxidizes aldehydes to carboxylic acids but does not react with ketones. Therefore, bromine water can indeed differentiate between an aldose (which contains an aldehyde group) and a ketose. Therefore, this statement is correct.

Analyzing Statement 2

Statement 2 mentions that all monosaccharides, whether aldose or ketose, are reducing sugars. Aldoses are reducing sugars due to their free aldehyde group. Ketoses can undergo keto-enol tautomerism, which allows them to behave as reducing sugars because they can form an aldehyde form. Thus, this statement is correct.

Analyzing Statement 3

Statement 3 states that osazone formation destroys the configuration about C-2 of an aldose, but not the rest of the molecule. Indeed, osazone formation reacts specifically at the C-1 and C-2 carbons, converting both into identical groups, which destroys the specific stereochemistry at C-2 while preserving the stereochemistry of other chiral centers in the molecule. This statement is correct.

Analyzing Statement 4

Statement 4 claims that a pair of diastereomeric aldoses differing only in configuration about C-2 are termed anomers. Anomers actually differ at the anomeric carbon (C-1 in aldoses), not C-2. Because diastereomers that differ only at C-2 are not anomers, this statement is incorrect.

Determining Correct Statements

Based on the analysis, Statements 1, 2, and 3 are correct. Statement 4 is incorrect because it misidentifies the characteristic of anomers.

Key concepts

Aldose

Aldoses are a type of carbohydrates known for containing an aldehyde group (-CHO) at the terminal carbon. This characteristic aldehyde group gives aldoses their reducing properties. In an aldose, the carbonyl group is located at the end of the carbon chain. Aldoses are crucial in biochemical processes because they can easily participate in oxidation-reduction reactions.
**Examples of Aldoses:** - Glucose - Galactose
The aldehyde group of aldoses can be oxidized by various agents, such as bromine water. Bromine water is an oxidizing agent that can distinguish aldoses from ketoses by oxidizing the aldehyde group of aldoses to form carboxylic acids.
This unique property not only helps in distinguishing them from ketoses but also plays a pivotal role in their behavior as reducing sugars. Understanding how aldoses function can be fundamental for those studying carbohydrate reactions and characterizations.

Ketose

Ketoses are another variety of carbohydrates that contain a ketone group (C=O) usually located at the second carbon in the carbon chain. This is different from aldoses, where the carbonyl is at the end of the molecule. Though ketoses do not have a free aldehyde group like aldoses, they are still considered reducing sugars.
**How Ketoses Become Reducing Sugars:** - They can undergo a chemical process called keto-enol tautomerism.
This process temporarily transforms the ketone into an aldehyde group, enabling them to act as reducing agents under certain conditions. Ketoses include sugars such as fructose.
Despite their ability to act as reducing sugars, bromine water does not react directly with ketoses as it does with aldoses. This makes bromine water a useful tool in differentiating between these two types of sugars. Ketoses play an essential role in energy metabolism and various biological pathways.

Reducing sugars

Reducing sugars are types of sugars that can donate electrons to another chemical, which means they can 'reduce' that chemical. This is most often demonstrated by their ability to reduce mild oxidizing agents, such as Benedict's or Fehling's reagents. Reducing sugars include all monosaccharides—both aldoses and ketoses—because they contain either a free aldehyde group or have the ability to form one through keto-enol tautomerism.
**Characteristics of Reducing Sugars:** - They have free aldehyde or ketone groups. - They can react with certain chemicals to form a color change, proving their presence.
This property is utilized in various test methods to differentiate them from non-reducing sugars like sucrose, which does not have a free aldehyde or ketone group. Understanding this concept is crucial in fields like food chemistry and biochemistry, where it is important to identify and differentiate between reducing and non-reducing sugars.

Osazone formation

Osazone formation is a chemical reaction used in carbohydrate chemistry to help identify sugars. This reaction involves the sugar reacting with phenylhydrazine, which converts the carbonyl groups (C=O) at the first two carbon positions into a crystalline osazone.
**Key Points of Osazone Formation:** - Targets the C-1 and C-2 carbons of a sugar molecule. - Changes the stereochemistry at C-2, but not at other positions.
Because it specifically affects only C-1 and C-2 during the reaction, it helps scientists to characterize sugars that might otherwise be difficult to distinguish. This characteristic change in structure does not affect other stereocenters in the sugar molecule.
By forming osazones, researchers can identify characteristic shapes and properties associated with different sugars, aiding in their identification and study. This process is vital for understanding the behavior and structure of carbohydrates in both biological and chemical contexts.