Problem 151 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

D-glucose and D-fructose all form the same osazone derivative because all of them have (a) the same configuration at \(\mathrm{C}-5\) (b) the same constitution at \(\mathrm{C}-1\) and \(\mathrm{C}-2\) (c) the same constitution (d) the same constitution and configuration at \(\mathrm{C}-3, \mathrm{C}-4, \mathrm{C}-5\) and \(\mathrm{C}-6\) but different constitution and configuration at \(\mathrm{C}-1\) and \(\mathrm{C}-2\) which becomes identical by osazone formation

Step-by-Step Solution

Answer

The correct answer is (d).

Step 1: Understanding Osazone Formation

Osazones are derivatives formed when reducing sugars react with phenylhydrazine. During this reaction, the carbonyl group at the anomeric carbon (1) and the adjacent carbon (2) are included in the formation of three phenylhydrazone bonds, resulting in the disappearance of the carbonyl group signatures at these positions.

Step 2: Comparing the Structures

The osazone formation does not alter the structure from 3 onwards. Therefore, if two sugars form the same osazone, they must have the same structure from 3 to the end. Specifically, the differences at 1 and 2 get neutralized during the osazone formation process.

Step 3: Analyzing the Relationship of D-Glucose and D-Fructose

D-Glucose and D-Fructose differ in their structure primarily at the 1 and 2 positions, where D-Glucose has an aldehyde and D-Fructose has a ketone. The transformation into osazones renders them identical at these positions because this area is where the reaction happens.

Step 4: Evaluating the Options

Option (a) is incorrect because the osazone formation doesn't depend on 5 configuration. Option (b) is incorrect as it refers to constitution rather than configuration adjustment. Option (c) is incorrect because the constitution is not preserved across all carbons. Option (d) is correct as it correctly states the preservation of the constitution and configuration from 3 onwards, and how 1 and 2 become identical upon osazone formation.

Key Concepts

D-Glucose

D-Glucose is one of the most common naturally occurring sugars. It's an aldohexose, meaning it has six carbon atoms and an aldehyde group. The structure of D-Glucose is characterized by its ability to form a ring. This cyclic form is due to the reaction between the aldehyde group at C-1 and the hydroxyl group at C-5, creating a hemiacetal linkage.
D-Glucose is critical in biology, serving as a major source of energy for cells. When broken down, it releases energy, which is stored as ATP (adenosine triphosphate), the energy currency of cells. It's also a building block for more complex carbohydrates like starch, cellulose, and glycogen.
The structure of D-Glucose can exhibit optical activity due to chiral centers, which are carbon atoms bonded to four different groups. This makes D-Glucose the "right-handed" version, or dextrorotatory, rotating plane-polarized light to the right. Understanding these features is crucial in studying how D-Glucose behaves in chemical reactions such as osazone formation.

D-Fructose

D-Fructose is known as a ketohexose because it contains a ketone group and six carbon atoms. This sugar is often found alongside glucose in fruits and is considered the sweetest of natural sugars.
Its structure differs from glucose primarily at positions C-1 and C-2. While glucose has an aldehyde group at these positions, fructose has a ketone group. This structural difference leads to different properties and reactions when fructose is subjected to chemical processes.
However, like glucose, D-Fructose can cyclize. This is a common feature in sugars where an internal reaction forms a five or six-membered ring. In the case of fructose, this involves the keto group and the hydroxyl group at C-6. Despite its structural differences, fructose can undergo similar reactions as glucose, such as osazone formation, where it ultimately behaves like glucose when analyzed from C-3 onwards.

Phenylhydrazine Reaction

The phenylhydrazine reaction is a method used to study the structure of sugars. It involves reacting sugars with phenylhydrazine to form derivatives called osazones. This reaction is specifically significant because it targets the carbonyl group at C-1 and C-2 in sugar molecules, changing them to form a triple bond with three phenylhydrazone groups.

• The reaction proceeds by first forming a phenylhydrazone derivative, followed by an intermediate known as a dihydrazone.
• Subsequently, an additional phenylhydrazine molecule reacts with one of the hydrazones, forming the final osazone structure.

One of the fascinating aspects of this reaction is that it masks any structural differences in sugars at C-1 and C-2. When applied to both glucose and fructose, the two sugars, which initially differ at these positions, become indistinguishable as they form identical osazones. This transformation provides insight into sugar molecules and facilitates their identification and differentiation in studies focusing on sugar metabolism and synthesis.