Problem 81 - A Complete Resource Book in Chemistry for JEE Main

Question

In the following reaction: Glucose \(+3 \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}-\mathrm{NH}_{2} \stackrel{\mathrm{H}^{\prime} /{ }}{\longrightarrow}\) Osazone \(+\mathrm{X}+\mathrm{Y}\) (X) and (Y) are (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) and \(\mathrm{NH}_{2} \mathrm{OH}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) and \(\mathrm{NH}_{3}\) (c) \(\mathrm{NH}_{2} \mathrm{OH}\) and \(\mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}-\mathrm{NHOH}\) and \(\mathrm{NH}_{3}\)

Step-by-Step Solution

Answer

The correct answer is (b) \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2} \) and \( \mathrm{NH}_{3} \).

Step 1: Understand the Reaction

In this reaction, glucose reacts with three moles of phenylhydrazine (\( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}-\mathrm{NH}_{2} \)) to form an osazone, along with byproducts \( X \) and \( Y \). Our goal is to identify \( X \) and \( Y \).

Step 2: Determine the Mechanism

The reaction of glucose with phenylhydrazine involves the conversion of the carbonyl groups of glucose into hydrazones and further into an osazone. Glycosidic bond cleavage and hydroxyl group replacement occur, leading to the formation of water (\( \mathrm{H}_{2} \mathrm{O} \)) and ammonia (\( \mathrm{NH}_{3} \)).

Step 3: Identify the Byproducts

Considering that one molecule of water is formed for each molecule of phenylhydrazine, and also some ammonia is released during the reaction, our byproducts \( X \) and \( Y \) are likely to include \( \mathrm{NH}_{3} \) and \( \mathrm{H}_{2} \mathrm{O} \).

Step 4: Choose the Correct Option

Given the chemical transformations, the correct byproducts \( X \) and \( Y \) in the reaction are \( \mathrm{NH}_{3} \) and \( \mathrm{H}_{2} \mathrm{O} \). However, the provided choices do not list \( \mathrm{H}_{2} \mathrm{O} \). The products \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2} \) and \( \mathrm{NH}_{3} \) match chemically with expected byproducts from similar reactions due to the excess of phenylhydrazine, corresponding to choice (b).

Key Concepts

Glucose Reaction Mechanism

When glucose reacts with phenylhydrazine, it undergoes a multi-step chemical change to form an osazone. The initial step involves the interaction of the aldehyde group of glucose with phenylhydrazine. This reaction turns the carbonyl group present in glucose into a hydrazone.
This hydrazone is not the final product. It undergoes further transformation in the presence of excess phenylhydrazine to form an osazone. During these transformations, other chemical reactions like glycosidic bond cleavage and hydroxyl group replacements occur. You might find the process a bit complex, but every change plays a crucial role.
In simpler words, think of it like shaping a piece of clay. The starting shape (carbonyl group) is molded several times (reactions) to become a hydrazone and finally an osazone. Each modification requires chemical energy and interaction with molecules like phenylhydrazine. It’s these changes that lead us to the formation of byproducts too, like ammonia, and sometimes trace amounts of water under certain conditions.

Phenylhydrazine in Chemistry

Phenylhydrazine is an important reagent in organic chemistry, especially for its role in the formation of osazones. When phenylhydrazine reacts with glucose, the reagent showcases its ability to interact with carbonyl compounds. This makes it quite unique.
Phenylhydrazine typically has a dual role in the reaction. Not only does it react with the carbonyl group to form hydrazones, but it also participates in the continued reaction to facilitate the formation of osazones. This is achieved by providing energy and facilitating the rearrangement of chemical bonds and groups.
Another remarkable aspect of phenylhydrazine in these reactions is its ability to act as a reagent repeatedly. Due to its excess availability in the reaction environment, it often results in overreaction, leading to various byproducts. This characteristic is why phenylhydrazine finds extensive use in laboratory settings, allowing chemists to identify specific sugars based on their osazone crystal formations.

Byproducts in Organic Reactions

Byproducts are substances formed during a chemical reaction that are not the primary product. In the reaction between glucose and phenylhydrazine, we deal with byproducts like ammonia and sometimes water. These arise due to the multi-step transformation process which the reactants undergo.
Typically, byproducts are an indication of a side process happening within the reaction. In our scenario, byproducts occur due to:

• Cleavage of glycosidic bonds.
• Hydroxyl group replacement.
• Excess presence of phenylhydrazine complicating the reaction further.

Understanding byproducts is crucial since they provide insights into the complete reaction mechanism and efficiency. They help in identifying conditions like excess reagent presence causing deviations from the intended pathway. In educational settings, paying attention to byproducts teaches students to think critically about all parts of a reaction, not just the final product.