Qualitative analysis of organic compounds is performed by Lassaigne's test by fusion with metallic sodium, by which the covalent compounds are converted into ionic compounds. Extra elements like `N,S,P` and halogens are detected by their usual tests.
Q. Prussian blue colour in the detection of nitrogen in lassaigne's test in due to the formation of:

Options

A. `Fe_2[Fe(CN)_6]`
B. `Fe_4[Fe(CN)_6]_3`
C. `Fe_3(CH)_6]_2`
D. `Fe[Fe(CN)_6]`

Answer

To solve the question regarding the Prussian blue color in the detection of nitrogen in Lassaigne's test, we can break down the process step by step:

### Step 1: Understanding Lassaigne's Test
Lassaigne's test is a qualitative analysis method used to detect the presence of certain elements in organic compounds, particularly nitrogen, sulfur, phosphorus, and halogens. In this test, the organic compound is fused with metallic sodium, which helps convert covalent compounds into ionic compounds.

**Hint:** Remember that the fusion with sodium is crucial for the conversion of covalent compounds to ionic forms.

### Step 2: Fusion with Sodium
When an organic compound containing nitrogen is fused with sodium, sodium cyanide (NaCN) is formed. This is because the sodium reacts with carbon and nitrogen in the organic compound to produce sodium cyanide.

**Hint:** The formation of sodium cyanide is a key step in detecting nitrogen.

### Step 3: Preparation of Sodium Extract
The product of the fusion, which is sodium cyanide, is then dissolved in water to prepare what is known as the sodium extract or Lassaigne's extract.

**Hint:** The sodium extract is where we will perform further tests to detect nitrogen.

### Step 4: Reaction with Iron(II) Sulfate
To detect nitrogen, we add freshly prepared iron(II) sulfate (FeSO4) to the sodium extract, followed by sodium hydroxide (NaOH). This results in the formation of a green precipitate of ferrous hydroxide (Fe(OH)2).

**Hint:** The green precipitate indicates the presence of iron(II) ions.

### Step 5: Formation of Ferrous Cyanide
The ferrous hydroxide then reacts with sodium cyanide to form ferrous cyanide (Fe(CN)2). This reaction is crucial as it leads to the next step.

**Hint:** The transition from ferrous hydroxide to ferrous cyanide is essential for the color change.

### Step 6: Formation of Prussian Blue
The ferrous cyanide can further react with excess sodium cyanide to form a complex known as sodium ferrocyanide (Na4[Fe(CN)6]). When this sodium ferrocyanide is treated with iron(III) chloride (FeCl3), it forms a deep blue complex known as Prussian blue (Fe4[Fe(CN)6]3).

**Hint:** The appearance of the blue color is a direct indication of the presence of nitrogen in the original organic compound.

### Conclusion
Thus, the Prussian blue color observed in the detection of nitrogen in Lassaigne's test is due to the formation of the complex **Fe4[Fe(CN)6]3**, also known as ferric ferrocyanide or Prussian blue.

**Final Answer:** The Prussian blue color in the detection of nitrogen in Lassaigne's test is due to the formation of **Fe4[Fe(CN)6]3**.