Problem 92 - A Complete Resource Book in Chemistry for JEE Main

Question

In the detection of sulphur, sodium nitroprusside solution is added to the Lassaigne's test solution, a purple coloured is obtained. It is due to (a) \(\mathrm{Na}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{5}\right.\) NOSNa] (b) \(\mathrm{Na}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\) (c) \(\mathrm{Fe}(\mathrm{CNS})_{3}\) (d) \(\mathrm{Na}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{5} \mathrm{NS}\right]\)

Step-by-Step Solution

Answer

The purple color is due to (d) \(\mathrm{Na}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{5} \mathrm{NS}\right]\).

Step 1: Understanding the Reaction Components

The problem involves the reaction between sodium nitroprusside and a test solution resulting from Lassaigne's test. When sodium nitroprusside is added to an aqueous solution obtained from heating an organic compound with sodium, the presence of sulfur leads to a specific color change.

Step 2: Identifying the Key Reaction

In the Lassaigne's test, an organic compound is fused with sodium to detect various elements, including sulfur. If sulfur is present in the compound, it forms sodium thiocyanate with the solution. When sodium nitroprusside is added to this solution, the presence of sulfur is indicated by the development of a purple color.

Step 3: Analyzing Reaction Outcomes

The purple coloration indicates a complex formation between the sulfur-containing ion and sodium nitroprusside. The most commonly known reaction involves the formation of a complex with formula: \[\text{Na}_3\left[\text{Fe}\left(\text{CN}\right)_5\text{NS}\right]\]This complex is responsible for the purple color observed.

Step 4: Matching the Reaction with Given Options

From the options provided, we identify:(a) \(\text{Na}_{3}\left[\text{Fe}(\text{CN})_{5}\right.\text{NOSNa}]\)(b) \(\text{Na}_{3}\left[\text{Fe}(\text{CN})_{6}\right]\)(c) \(\text{Fe}(\text{CNS})_{3}\)(d) \(\text{Na}_{3}\left[\text{Fe}(\text{CN})_{5} \text{NS}\right]\) Option (d) corresponds to the correct complex that forms a purple coloration.

Key Concepts

Sulfur Detection

The detection of sulfur in an organic compound is a significant step in organic analysis. Lassaigne’s test is the go-to method for this process. In this test, the compound is fused with metallic sodium, converting various elements, including sulfur, into water-soluble sodium compounds. The key reaction here is the formation of sodium thiocyanate if sulfur is present. This part of the process is crucial as it allows the sulfur to be detected through further testing.
To confirm the presence of sulfur, a more complex reaction is performed where sodium nitroprusside is added. A noticeable change in color, typically to a purple hue, signifies the presence of sulfur. This specific color change is an indicator of sulfur-sodium thiocyanate in the solution.

Sodium Nitroprusside

Sodium nitroprusside is a chemical compound used extensively in qualitative inorganic analysis and tests like Lassaigne’s. When added to sodium thiocyanate, a vivid purple color appears.
This compound is uniquely structured, containing a central iron atom surrounded by cyanide groups and a nitrosyl group. It’s the ability of sodium nitroprusside to form complexes with certain ions that make it useful for sulfur detection in Lassaigne’s test. Its reactivity with sulfur compounds is what leads to the distinctive color change that students observe during experiments.

Complex Formation

Complex formation is a fundamental concept involved in sulfur detection using sodium nitroprusside. When sodium nitroprusside encounters sodium thiocyanate, formed if sulfur is part of the organic compound, it results in a complex with a notable structure.
The chemical equation illustrating this is: \[\text{Na}_3\left[\text{Fe}(\text{CN})_5\text{NS}\right]\]
This complex is what ultimately gives rise to the purple color. Complexes are stable chemical species formed from a central metal atom bonded to molecules or ions. In this instance, the iron atom within sodium nitroprusside is crucial for the stability and formation of the color-signifying complex.

Organic Compound Analysis

Analyzing organic compounds is a meticulous process that frequently involves determining the presence of key elements such as sulfur, nitrogen, and halogens. Lassaigne’s test is an age-old procedure utilized for these purposes.
In organic compound analysis, it’s essential to transform the molecules into a form that is more identifiable, which is what the Lassaigne’s test achieves with the sodium fusion process.

• First, the compound’s elements are converted into ionic forms.
• This transformation allows further tests to accurately detect elements like sulfur.

Through the fusion of the organic compound with sodium, followed by the addition of specific reagents such as sodium nitroprusside, the detailed composition of the compound can be revealed.