Question

In Lassaigne’s extract of an organic compound, both nitrogen and sulphur are present, which gives blood red colour with Fe³⁺ due to the formation of

• A. NaSCN
• B. [Fe(CN)₅ NOS]^4-
• C. [Fe(SCN)]²⁺
• D. Fe₄[Fe(CN₆)]₃.xH₂O

Answer 1

The Correct Option is C

To determine the compound responsible for the blood red color when both nitrogen and sulfur are present in Lassaigne's extract, let's analyze the chemical process involved.

Lassaigne’s test is used to detect the presence of halogens, nitrogen, and sulfur in organic compounds. When the compound is fused with sodium metal, and then dissolved in water, various reactions can help identify these atoms.

When both nitrogen and sulfur are present in the compound, the following reaction typically occurs: sodium from the fusion reacts to form sodium thiocyanate (NaSCN). The equation for this reaction is:

\(NaCNS + Fe^{3+} \rightarrow [Fe(SCN)]^{2+} + other\ products\)

In the presence of ferric ions \((Fe^{3+})\), sodium thiocyanate forms a complex iron thiocyanate, \([Fe(SCN)]^{2+}\), which presents a distinct blood red color.

Let's evaluate each option:

• NaSCN: This is the intermediate thiocyanate compound which reacts with Fe³⁺ to form the red complex, but NaSCN itself does not cause the blood red color.
• [Fe(CN)₅ NOS]^4-: This is a different coordination complex, not related to the red color formation.
• [Fe(SCN)]²⁺: This is the iron thiocyanate complex responsible for the blood red color. This is the correct answer.
• Fe₄[Fe(CN)₆]₃.xH₂O: Known as Prussian blue, this complex is not related to this specific reaction.

Thus, the correct answer is \([Fe(SCN)]^{2+}\), which is responsible for the blood red color observed in Lassaigne's test when both nitrogen and sulfur are present.