Question

Explain the products obtained when (i) Fructose is treated with excess phenyl hydrazine (ii) Glucose is heated with concentrated hydrochloric acid (iii) Glucosazone is hydrolysed with dilute hydrochloric acid

Short answer

Answer: (i) Fructosazone (ii) 5-(hydroxymethyl)furfural (iii) Glucose and phenyl hydrazine

Step-by-step solution

(Reaction 1: Fructose with Phenyl Hydrazine)

We are given that fructose is treated with excess phenyl hydrazine. When fructose reacts with phenyl hydrazine, it forms fructose phenylhydrazone by losing water molecules and eliminates the carbonyl group. If excess phenyl hydrazine is present, fructose phenylhydrazone further undergoes reaction to form osazone. The product of this reaction is called fructosazone.

(Reaction 2: Glucose with Concentrated Hydrochloric Acid)

In this reaction, we heat glucose with concentrated hydrochloric acid. The concentrated hydrochloric acid acts as a dehydrating agent and converts glucose into a five-membered ring compound called 5-(hydroxymethyl)furfural. This is because concentrated hydrochloric acid helps in the dehydration and rearrangement of glucose.

(Reaction 3: Glucosazone with Dilute Hydrochloric Acid)

In the third reaction, glucosazone is hydrolyzed with dilute hydrochloric acid. When glucosazone undergoes hydrolysis, it decomposes to form glucose and phenyl hydrazine. This is because the bond between the glucose moiety and the phenyl hydrazine moiety in glucosazone breaks due to the presence of dilute hydrochloric acid which acts as an acid catalyst. To summarize: 1. When fructose is treated with excess phenyl hydrazine, fructosazone is formed. 2. When glucose is heated with concentrated hydrochloric acid, 5-(hydroxymethyl)furfural is formed. 3. When glucosazone is hydrolysed with dilute hydrochloric acid, glucose and phenyl hydrazine are formed.

Key concepts

Fructose and Phenyl Hydrazine Reaction

When fructose interacts with phenyl hydrazine, an interesting reaction occurs. Initially, fructose reacts with phenyl hydrazine to form a compound known as fructose phenylhydrazone. This transformation involves the removal of water molecules and elimination of the carbonyl group.

But the reaction doesn't stop there. In the presence of an excess of phenyl hydrazine, fructose phenylhydrazone undergoes a further reaction to form fructosazone. This osazone formation is an important reaction for identifying sugars since osazones from different sugars often have distinct shapes or forms.

Thus, the process leads to the creation of fructosazone, providing a way to distinguish fructose from other sugars.

Glucose Dehydration

When glucose is heated with concentrated hydrochloric acid, a transformation takes place. Concentrated hydrochloric acid acts primarily as a dehydrating agent in this process.

As a result of the dehydration, glucose undergoes rearrangement and transforms into a compound identified as 5-(hydroxymethyl)furfural (HMF). This five-membered ring structure is a key intermediate in various chemical processes and holds industrial significance.

The creation of 5-(hydroxymethyl)furfural from glucose is a classic example of how sugars can undergo structural rearrangements through dehydration, impacting both scientific and industrial fields.

Glucosazone Hydrolysis

In the case of glucosazone hydrolysis, the presence of dilute hydrochloric acid plays a pivotal role. When glucosazone is exposed to dilute hydrochloric acid, it undergoes hydrolysis.

This reaction involves breaking the bond between the glucose and phenyl hydrazine moieties within glucosazone. As a result, the compounds that emerge from this breakdown are glucose and phenyl hydrazine.

This step highlights the effectiveness of dilute hydrochloric acid as an acid catalyst, prompting glucosazone to decompose back to its constituent parts. Such reactions are critical for studying the characteristics and behaviors of sugar derivatives.