Problem 7 - A Complete Resource Book in Chemistry for JEE Main

Question

In Lassaigne's test for nitrogen, the blue colour is due to the formation of (a) ferriferrocyanide (b) sodium cyanide (c) potassium fericyanide (d) sodium ferrocyanide

Step-by-Step Solution

Answer

(a) Ferriferrocyanide

Step 1: Understanding Lassaigne's Test

Lassaigne's test is a qualitative analysis used to detect the presence of nitrogen, sulfur, and halogens in an organic compound. In the test for nitrogen, the organic compound is fused with sodium to form sodium cyanide, which then reacts with a ferric iron solution to produce Prussian blue.

Step 2: Formation of Sodium Cyanide

When the organic compound is heated with metallic sodium, any nitrogen present is converted to sodium cyanide ( ext{NaCN}). This sodium cyanide will be an important intermediate in forming the colored compound.

Step 3: Reaction to Form Prussian Blue

The sodium cyanide formed is treated with an iron(II) salt solution. The iron(II) ions react with sodium cyanide to form sodium ferrocyanide ( ext{Na}_4[ ext{Fe}( ext{CN})_6]). This compound then reacts with a ferric iron solution.

Step 4: Formation of Ferriferrocyanide

In the final step, sodium ferrocyanide reacts with ferric ions to form ferriferrocyanide ( ext{Fe}_4[ ext{Fe}( ext{CN})_6]_3), commonly known as Prussian blue, which imparts the characteristic blue color.

Key Concepts

Nitrogen Detection

Nitrogen detection in organic compounds is a crucial step in analytical chemistry, especially when performing Lassaigne's test. This test is named after its inventor, Jules Lassaigne. The primary aim here is to qualitatively detect nitrogen in organic samples. By heating the sample with metallic sodium, any nitrogen present is converted to sodium cyanide (\( \text{NaCN} \)). This is achieved because sodium reacts with the nitrogen in the organic matrix to break bonds and form the cyanide ion, which is a complex anion combined with sodium to form sodium cyanide.

Using Lassaigne's test doesn't only apply to nitrogen; it's also effective in detecting sulfur and halogens. However, in the context of nitrogen, this test is efficient due to the simple chemical transformation and the subsequent reactions that lead to an easily identifiable color change.

Sodium Cyanide Formation

The formation of sodium cyanide (\( \text{NaCN} \)) is a critical intermediary step in Lassaigne's test for nitrogen. This compound is created when an organic sample containing nitrogen is fused or intensely heated with a piece of metallic sodium.

Here's how it works:

• The bonding in the organic compound is broken down under heat and pressure, facilitating the nitrogen atoms to react with sodium ions.
• This reaction turns nitrogen atoms into the cyanide ion (\( \text{CN}^- \)).
• These cyanide ions then pair with sodium ions to form sodium cyanide.

This sodium cyanide is pivotal for the subsequent formation of Prussian blue, as it serves as a link to ferric iron, transforming into a deeply colored compound.

Prussian Blue Reaction

The Prussian blue reaction is the final and visually identifiable step in Lassaigne's test for nitrogen. It culminates in the iconic blue color, signifying the presence of nitrogen.

Here's a rundown of the transformation:

• Sodium cyanide initially formed contacts iron(II) salts, yielding sodium ferrocyanide (\(\text{Na}_4[\text{Fe}(\text{CN})_6] \)).
• Subsequently, this complex reacts with ferric ions present in a ferric chloride solution.
• This reaction leads to the production of ferriferrocyanide (\(\text{Fe}_4[\text{Fe}(\text{CN})_6]_3 \)), also known as Prussian blue.

Prussian blue's intense color is due to the electron transfer between the iron ions in the compound's structure. This transfer is responsible for the dramatic blue hue which is distinctly visible, making it an effective indicator of nitrogen in the test.