Question

In Lassaigne’s test, when both nitrogen and sulphur are present in an organic compound, a blood-red colour is observed upon adding FeCl$_3$ to the sodium fusion extract. This colour is due to the formation of X. What is X?

• A. NaSCN
• B. [Fe(SCN)]$^{2+}$
• C. [Fe(CN)$_5$NOS]$^{4-}$
• D. [Fe(SCN)$_2$]$^{2+}$

Hint:

In Lassaigne’s test, a blood-red color indicates the presence of both nitrogen and sulfur, forming \( \text{NaSCN} \), which then reacts with \( \text{Fe}^{3+} \) to produce the \( [\text{Fe(SCN)}]^{2+} \) complex. Ensure the complex matches the observedwarden’t observed color and charge.

Answer 1

The Correct Option is B

Step 1: Understand Lassaigne’s Test for Nitrogen and Sulfur
Lassaigne’s test is used to detect elements like nitrogen, sulfur, and halogens in an organic compound by fusing the compound with sodium metal, creating a sodium fusion extract. When both nitrogen and sulfur are present in the organic compound, they react with sodium to form sodium thiocyanate (\( \text{NaSCN} \)) during the fusion process:
\[ \text{C} + \text{N} + \text{S} + \text{Na} \rightarrow \text{NaSCN} \] Step 2: Reaction with FeCl$_3$ and Formation of the Blood-Red Color
When \( \text{FeCl}_3 \) is added to the sodium fusion extract containing \( \text{NaSCN} \), the \( \text{Fe}^{3+} \) ions react with the thiocyanate ions (\( \text{SCN}^- \)) to form a blood-red colored complex. The reaction is:
\[ \text{Fe}^{3+} + \text{SCN}^- \rightarrow [\text{Fe(SCN)}]^{2+} \] The complex \( [\text{Fe(SCN)}]^{2+} \) is responsible for the characteristic blood-red color observed in the test. This is a well-known qualitative test for the presence of thiocyanate ions, which indicates that both nitrogen and sulfur were present in the original organic compound. Step 3: Consider the Coordination Number and Charge
\( \text{Fe}^{3+} \) (from \( \text{FeCl}_3 \)) has a +3 charge.
\( \text{SCN}^- \) has a –1 charge.
The complex \( [\text{Fe(SCN)}]^{2+} \) has a net charge of +2 (\( +3 - 1 = +2 \)).
In aqueous solution, \( \text{Fe}^{3+} \) often forms complexes with \( \text{SCN}^- \), and while higher coordination complexes like \( [\text{Fe(SCN)}_6]^{3-} \) can form with excess \( \text{SCN}^- \), the blood-red color is typically attributed to \( [\text{Fe(SCN)}]^{2+} \) in the context of Lassaigne’s test. Step 4: Evaluate the Options
Option (1): NaSCN. This is the intermediate formed during sodium fusion, but it’s not the blood-red complex formed after adding \( \text{FeCl}_3 \). \( \text{NaSCN} \) is colorless.
Option (2): [Fe(SCN)]$^{2+$.} This is the correct complex formed when \( \text{Fe}^{3+} \) reacts with \( \text{SCN}^- \), producing the blood-red color.
Option (3): [Fe(CN)$_5$NOS]$^{4-$.} This complex involves cyanide (\( \text{CN}^- \)) and nitrosyl (\( \text{NO} \)), which are not relevant to Lassaigne’s test for nitrogen and sulfur. This is incorrect.
Option (4): [Fe(SCN)$_2$]$^{2+$.} While \( \text{Fe}^{3+} \) can form complexes with multiple \( \text{SCN}^- \) ligands, the charge would be \( +3 - 2 = +1 \), not +2 as shown. Moreover, the blood-red color is typically attributed to \( [\text{Fe(SCN)}]^{2+} \), not a higher coordination complex in this context. Step 5: Analyze Options
Option (1): NaSCN. Incorrect, as \( \text{NaSCN} \) is not the colored complex; it’s an intermediate.
Option (2): [Fe(SCN)]$^{2+$.} Correct, as it matches the blood-red complex formed in Lassaigne’s test.
Option (3): [Fe(CN)$_5$NOS]$^{4-$.} Incorrect, as this complex is unrelated to the test.
Option (4): [Fe(SCN)$_2$]$^{2+$.} Incorrect, as the charge is wrong, and the blood-red color is due to \( [\text{Fe(SCN)}]^{2+} \).