Test for Halogens in Organic Compounds

Halogens present in an organic compound is detected by Lassaigne’s test. The element present in the compound is converted from covalent form into the ionic form by fusing the compound with sodium metal.

Halide of sodium so formed on sodium fusion is extracted from the fused mass by boiling it with distilled water. This extract is known as sodium fusion extract.

The sodium fusion extract is acidified with nitric acid and then treated with silver nitrate. A white precipitate, soluble in ammonium hydroxide shows the presence of chlorine, a yellowish precipitate, sparingly soluble in ammonium hydroxide shows the presence of bromine and a yellow precipitate, insoluble in ammonium hydroxide shows the presence of iodine.

If nitrogen or sulphur is also present in the compound, the sodium fusion extract is first boiled with concentrated nitric acid to decompose cyanide or sulphide of sodium formed during Lassaigne’s test. These ions would otherwise interfere with silver nitrate test for halogens.