Question

In which of the following compound(s) nitrogen can be detected by Lassaigne’s test?

• A.
• B.
• C.
• D.

Answer 1

Answer: (A), (C), (D)

Lassaigne’s test can be carried out with the help of sodium fusion extract.
In this test, the elements present in a compound are converted from covalent to ionic form by fusing the organic compound with Sodium (Na).
$Na+C+N\stackrel{\Delta }{⟶}NaCN$
Cyanides of sodium formed on sodium fusion are extracted from the fused mass by boiling with water.
This extract is known as Sodium Fusion Extract (Lassaigne Extract).
This extract is used for detection of N.

For the formation of cyanide ion, the compound must have carbon atoms.
In the given options, compound (a), (c) and (d) have both carbon atoms and nitrogen so they will form cyanide ion and gives positive Lassaigne's test.
Compound (b) doesn't have a carbon atom so it won't give positive Lassaigne's test for detection of nitrogen
Theory :
Detection of nitrogen:
Lassaigne extract (having NaCN) is heated with Iron(II) sulphate to form sodium hexacyanoferrate (II)
$F{e}^{2+}+6NaCN\stackrel{\Delta }{⟶}\underset{\text{Sodium hexacyanoferrate (II)}}{N{a}_4\left[Fe\right(CN{)}_6]}$

On heating with concentrated sulphuric acid, some Fe(II) ions are oxidised to Fe(III).
$F{e}^{2+}\underset{H^{+}}{\overset{\Delta }{\to }}F{e}^{3+}$

Fe(III) reacts with Sodium hexacyanoferrate(II) to produce prussian blue colour of iron(III) hexacyanoferrate(II) which confirms the presence of nitrogen.
$3\left[Fe\right(CN{)}_6{]}^{4-}+4F{e}^{3+}\stackrel{x{H}_{2}O}{⟶}\underset{\text{Indicates the presence of N}}{F{e}_4\left[Fe\right(CN{)}_6{]}_3.x{H}_{2}O}$
Common name of this product is ferriferrocyanide