Question

In Lassaigne’s test, violet colour is formed when sodium fusion extract of an organic compound is treated with solution of ‘X’. What is ‘X’?

• A. \(\mathrm{Pb(CH_3COO)_2}\)
• B. \(\mathrm{Na_3[Fe(CN)_5NOS]}\)
• C. \(\mathrm{Na_4[Fe(CN)_6]}\)
• D. \(\mathrm{(NH_4)_2MoO_4}\)

Hint:

In Lassaigne’s test for sulphur: \[ \text{Sodium nitroprusside} \; \left(\mathrm{Na_3[Fe(CN)_5NOS]}\right) \] gives a characteristic violet or purple colour. Remember:
• Violet colour \(\rightarrow\) Sodium nitroprusside test
• Black precipitate \(\rightarrow\) Lead acetate test

Answer 1

The Correct Option is B

Concept: Lassaigne’s test is used for the detection of extra elements such as nitrogen, sulphur and halogens in organic compounds. When an organic compound is fused with sodium metal, these elements are converted into water-soluble ionic compounds which can be detected by suitable reagents. For sulphur detection, sodium fusion converts sulphur into sodium sulphide: \[ 2Na + S \rightarrow Na_2S \] The sulphide ion present in the sodium fusion extract gives characteristic colour reactions with specific reagents.

Step 1: Understand the reagent used for sulphur detection. One of the most important confirmatory tests for sulphur involves sodium nitroprusside solution. The formula of sodium nitroprusside is \[ \mathrm{Na_3[Fe(CN)_5NOS]} \] When sodium fusion extract containing sulphide ions is treated with sodium nitroprusside solution, a violet or purple coloured complex is formed. \[ Na_2S + Na_3[Fe(CN)_5NOS] \longrightarrow \text{Violet coloured complex} \] Thus, appearance of violet colour confirms the presence of sulphur in the organic compound.

Step 2: Analyse the remaining options.
• \(\mathrm{Pb(CH_3COO)_2}\) produces a black precipitate of PbS with sulphide ions and is not responsible for the violet colour.
• \(\mathrm{Na_4[Fe(CN)_6]}\) is sodium ferrocyanide and is used in different qualitative tests.
• \(\mathrm{(NH_4)_2MoO_4}\) is ammonium molybdate, commonly used for phosphate detection. None of these reagents produce the characteristic violet colour in Lassaigne’s sulphur test.

Step 3: Identify X. The reagent that gives a violet colour with sodium fusion extract containing sulphur is sodium nitroprusside. \[ \boxed{\mathrm{Na_3[Fe(CN)_5NOS]}} \] Hence, \[ \boxed{\text{Option (B)}} \]