Question

Give the sequence of reactions when D-glucose reacts with excess of \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NHNH}_{2}\) in the presence of acetic acid?

Short answer

Answer: The sequence of reactions involves the formation of a phenylhydrazone followed by an osazone.

Step-by-step solution

Formation of phenylhydrazone

Initially, D-glucose reacts with phenylhydrazine to form a phenylhydrazone. During this reaction, the carbonyl group (C=O) on the glucose molecule reacts with phenylhydrazine, and water is released as a by-product. The reaction can be represented as: D-Glucose + Phenylhydrazine \(\rightarrow\) D-Glucose phenylhydrazone + H\(_2\)O

Formation of osazone by continuous reaction with phenylhydrazine

Since there is an excess of phenylhydrazine, the D-Glucose phenylhydrazone will continue to react with more phenylhydrazine, resulting in the second carbonyl group also being converted to a phenylhydrazone. Finally, these phenylhydrazones condense to form an osazone, with the release of two molecules of ammonia. This reaction can be represented as: D-Glucose phenylhydrazone + 2 Phenylhydrazine \(\rightarrow\) D-Glucose osazone (Phenyl-2,3,4,5-tetra-acetyl-n-hexosazone) + 2 NH\(_3\) The overall sequence of reactions when D-glucose reacts with excess phenylhydrazine in the presence of acetic acid includes the formation of a phenylhydrazone and then an osazone.

Key concepts

D-Glucose

D-Glucose is one of the most important sugars in carbohydrate chemistry. As an aldohexose, it means it contains six carbon atoms and an aldehyde group. This sugar is the primary energy source for cells and is a building block for many carbohydrates.

Its structure is significant in reactions due to the presence of the carbonyl (C=O) group, which makes it highly reactive with compounds like phenylhydrazine. This characteristic is crucial when observing how D-glucose interacts in different reactions, such as forming hydrazones and osazones. Understanding D-glucose's role in these chemical reactions helps grasp broader biochemical processes.

Phenylhydrazone

When D-glucose reacts with phenylhydrazine, a compound known as phenylhydrazone is formed. This reaction is an example of condensation, where the carbonyl group of D-glucose ( C=O ) reacts with phenylhydrazine (C 6 H 5 NNH 2 ), resulting in the release of a water molecule.

There are several points of importance in this reaction:

• The carbonyl group identifies the site on D-glucose where the reaction occurs.
• Condensation reactions, like this, are crucial in organic chemistry for forming larger, more complex molecules.
• The formation of phenylhydrazone stabilizes the molecule and facilitates further reactions, specifically leading to the osazone formation.

These attributes make phenylhydrazone formation a central step in studying carbohydrate reactions.

Osazone Formation

Osazone formation occurs when the phenylhydrazone of D-glucose continues to react with additional phenylhydrazine. In this process, both of the functional groups on the adjacent carbon atoms of D-glucose are involved.

Key aspects of osazone formation include:

• It involves further reaction with excess phenylhydrazine, highlighting why a large supply is used in the initial setup.
• Osazones possess distinct physical properties that make them useful for identifying sugars based on crystal formation.
• These reactions demonstrate how sugars can be differentiated through chemical reactions, providing insights into sugar structures and properties.

The resulting osazone has two phenylhydrazone groups, exemplifying the characteristic transformations within sugar chemistry.

Phenylhydrazine

Phenylhydrazine, with the formula C 6 H 5 NHNH 2 , is a critical reagent in carbohydrate chemistry, particularly for its role in forming phenylhydrazones and osazones.

Here are some important details about phenylhydrazine:

• It acts as a reducing and nucleophilic agent, transforming carbonyl groups in sugars like D-glucose.
• Its capability to react with ketones and aldehydes makes it versatile in preparing derivatives of various carbohydrates.
• Phenylhydrazine’s reactivity is instrumental in organic synthesis and analytical chemistry, where it's used to identify sugars by their osazone end-products.

Understanding phenylhydrazine’s properties thus provides insights into the chemical behavior of sugars and their derivatives.

Acetic Acid

Acetic acid is often used in these reactions as a mild acid catalyst that aids in the successful transformation of D-glucose to phenylhydrazone and eventually osazone. Though not a direct Reactant, its presence is pivotal in balancing the reaction environment.

Acetic acid provides:

• A proton-rich environment which facilitates the nucleophilic attack by phenylhydrazine on the carbonyl group.
• Enhanced reaction kinetics, allowing faster conversion rates.
• Stability of the intermediate compounds, ensuring that these do not degrade before completing the reaction sequence.

Hence, acetic acid plays a subtle yet crucial role in the successful execution of carbohydrate-related chemical transformations.