Problem 3 - A Complete Resource Book in Chemistry for JEE Main

Question

The compound that does not give a blue colour in Lassaigne's test is (a) aniline (b) glycine (c) hydrazine (d) urea

Step-by-Step Solution

Answer

Urea does not give a blue color in Lassaigne's test.

Step 1: Understanding Lassaigne's Test

Lassaigne's test is used to detect the presence of nitrogen, sulfur, and halogens in a compound. When nitrogen is present in the compound, it reacts with sodium and then with ferrous sulfate to form a Prussian blue color, which indicates the presence of nitrogen as a cyanide ion.

Step 2: Evaluate Aniline

Aniline is an aromatic amine containing nitrogen. During the Lassaigne's test, aniline can form a cyanide ion which reacts to give a blue color. Thus, aniline gives a blue color in Lassaigne's test.

Step 3: Evaluate Glycine

Glycine is an amino acid and contains nitrogen, which can form the cyanide ion upon reaction with sodium and ferrous sulfate, resulting in a blue color in Lassaigne's test.

Step 4: Evaluate Hydrazine

Hydrazine ( ext{N}_2 ext{H}_4) also contains nitrogen, and during Lassaigne’s test, it can form a cyanide ion. This would lead to a blue color indicative of the presence of nitrogen.

Step 5: Evaluate Urea

Urea ( ext{NH}_2 ext{CONH}_2) although it contains nitrogen, when subjected to Lassaigne’s test, does not produce the cyanide ion needed for the blue color. This is due to its structure, making it the compound that does not give a blue color.

Key Concepts

Understanding Aniline and Lassaigne's Test

Aniline is an organic compound primarily recognized as an aromatic amine. It's made up of a benzene ring attached to an amino group \(\text{NH}_2\). Aniline’s nitrogen content is essential when applying Lassaigne's Test, a classic test used to identify nitrogen in organic compounds. During this test:

• Aniline reacts with sodium, forming sodium cyanide (\(\text{NaCN}\)).
• This then interacts with ferrous sulfate to produce a distinct Prussian blue color.

This reaction indicates the presence of nitrogen as cyanide ions \(\text{CN}^-\) creating the notable blue color. Aniline is a great example of how nitrogen in aromatic amines can be successfully detected using Lassaigne’s Test.

Glycine’s Reaction in Lassaigne's Test

Glycine is the simplest amino acid, featuring an amino group \(\text{NH}_2\) and a carboxyl group \(\text{COOH}\). It contains nitrogen, making it a candidate for Lassaigne's Test in detecting this element. During the test:

• The nitrogen in glycine reacts with sodium to form sodium cyanide.
• The cyanide ions then react with ferrous sulfate to yield the characteristic Prussian blue color.

This reaction affirms the presence of nitrogen in glycine. Thus, it clearly shows the blue coloration associated with the presence of cyanide ions, showcasing how amino acids like glycine can interact in Lassaigne’s Test.

Hydrazine and Lassaigne's Test

Hydrazine, with the chemical formula \(\text{N}_2\text{H}_4\), is another nitrogen-containing compound evaluated in Lassaigne’s Test. Despite its simple structure, hydrazine's nitrogen plays a crucial role during the test:

• The compound reacts with sodium, eventually forming sodium cyanide, a facilitator for the test’s reactions.
• When combined with ferrous sulfate, hydrazine's cyanide ions help produce the blue color typical of the test.

Hydrazine is thus clearly identifiable in Lassaigne’s Test thanks to its efficient formation of cyanide ions that lead to the formation of the blue complex, differentiating it from other non-reactive compounds.

Why Urea Fails in Lassaigne’s Test

Urea \(\text{NH}_2\text{CONH}_2\) contains nitrogen, similar to many other organic compounds. However, its behavior in Lassaigne’s Test is unique:

• Urea does not form sodium cyanide when reacted with sodium. Instead, the structure of urea prevents the necessary chemical transformations.
• This absence of cyanide ions leads to no reaction with ferrous sulfate and, consequently, no blue coloration.

This makes urea the exception among nitrogen-containing structures in Lassaigne's Test, as it does not produce a Prussian blue color due to its inability to generate cyanide ions under the test conditions. Thus, urea is swiftly identifiable by its lack of reaction in the test, unlike compounds such as aniline, glycine, and hydrazine.