Question

In Lassaigne’s test, the organic compound is fused with a piece of sodium metal in order to:
a) Increase the ionisation of the compound
b) Decrease the melting point of the compound
c) Increase the reactivity of the compound
d) Convert the covalent compound into a mixture of ionic compounds

Answer 1

Hint: Organic compounds are bounded by covalent bonds. Any element other than carbon, hydrogen and oxygen can be detected by this method.

Complete answer:
Lassaigne’s test is also known as the Sodium fusion test. This test was developed by J.L. Lassaigne. This test is used in elemental analysis for the qualitative determination of the presence of foreign elements i.e. nitrogen, sulphur and halogens in an organic compound.
Organic compounds are bonded by covalent bonds. Any element other than carbon, hydrogen and oxygen is known as a foreign element. Its detection is possible only when these compounds are converted to its organic form.
Hence, the organic element is fused with a piece of sodium metal. It will convert the covalently bonded foreign element to its ionic form.
On fusion, the following reactions take place –
\[Na+C+N\to NaCN\]
\[2Na+S\to N{{a}_{2}}S\]
\[Na+X\to NaX\] [X= Cl, Br or I]
C, N, S and X come from the organic compound. The ionic compounds hence formed can be detected by simple chemical tests.
For example –
Test for Sulphur: Treating sodium fusion extract with sodium nitroprusside produces violet colour, therefore indicating presence of sulphur.
\[{{S}^{2-}}+{{[Fe{{(CN)}_{5}}NO]}^{2-}}\to {{[Fe{{(CN)}_{5}}NOS]}^{4-}}\]

Note: Cyanide, sulphide and halide of sodium so formed on sodium fusion are extracted from the fused mass by boiling it with distilled water. This extract is known as sodium fusion extract or Lassaigne’s extract.