Test: Analysis of Organic Compounds - NEET MCQ

Nitrogen, Sulphur, and halogens present in organic compounds are detected by Lassaigne’s test. Here, a small piece of Na metal is heated in a fusion tube with the organic compound.

If both nitrogen and sulphur are present in an organic compound, sodium fusion will convert it into sodium thiocyanate which then reacts with Fe³⁺ to form a blood colour complex [Fe(SCN)₃​].

• When a compound is heated with copper(ii) oxide it is the test for carbon.           
• These are detected by heating the compound with copper(II) oxide. Carbon is oxidized to CO₂ – confirmed by reaction with lime water that gives turbidity and turns limewater milky.
• Hydrogen is oxidized to H2O – tested with anhydrous copper sulphate which turns blue.

Hence, the correct answer is Option C.

The correct answer is option C
The halogens, nitrogen and sulphur are covalently bonded to the organic compounds. In order to detect them, the elements need to be converted into their ionic forms. This is done by fusing the organic compound with sodium metal. The ionic compounds formed during the fusion are extracted in aqueous solution and can be detected by simple chemical tests. The extract is called sodium fusion extract or Lassaigne's extract.

The phosphorus containing organic compound are detected by 'Lassaigne's test' by heated with an oxidizing agent (sodium peroxide)
The phosphorus present in the compound in oxidised to phosphate.
The solution is boiled with nitric acid and then treated with ammonium molybdate to produced canary yellow precipitate.
Na₃PO₄ + 3HNO₃ ⟶ H3PO₄ + 3NaNO₃ 
H₃PO₄ + 12(NH₄)2MoO₄ (Ammonium molybdate) + 21HNO₃⟶(NH₄)3PO4⋅12MoO₃↓ + 21NH₄NO₃ + 12H₂O

 If silver nitrate solution is poured into a solution of sodium chloride, a chemical reaction occurs forming a white precipitate of silver chloride.

Mass of Bromine = 80u

Mass of AgBr = 188u

Weight of AgBr = 0.12g
Weight of organic compound = 0.15g

The correct answer is option C
A pale yellow precipitate soluble in ammonium Hydroxide solution indicates the presence of Bromine.

In Lassaigne's test for the detection of nitrogen in an organic compound, the appearance of a blue coloured compound is due to ferric ferrocyanide.
 
In this test, sodium fusion extract is boiled with ferrous sulphate and then acidified with conc. sulphuric acid. Prussian blue colour complex ferric ferrocyanide confirms the presence of nitrogen.
FeSO₄+2NaOH→Fe(OH)₂+Na2SO₄
6NaCN+Fe(OH)₂→Na4Fe(CN)₆+2NaOH
 
3NaFe(CN)₆+4FeCl₃→Fe4[Fe(CN)₆]₃+12NaCl

It is ferric ferrocyanide.

Hence, the correct answer is Option D.