Question

Aniline and aliphatic amine can be distinguished by azo dye test.
A.True
B.False

Answer 1

The products formed when nitrous acid reacts with aliphatic or aromatic amines show different behaviours. Aniline is an aromatic amine due to the aromatic benzene ring attached to it.

Complete step by step answer:
When aromatic amines are reacted with nitrous acid at low temperature, the product formed is diazonium salt. Then, the coupling of diazonium salt with 2- naphthol in the presence of a base generally an aqueous solution of sodium hydroxide occurs, a bright coloured azo dye is formed as a product.
Whereas, if we react nitrous acid with aliphatic amines, we observe an effervescence of nitrogen gas and there is no change in colour.
For the azo dye test, we first prepare nitrous acid in situ by the reaction of sodium nitride with dilute hydrochloric acid. The nitrous acid thus prepared reacts with the amine group of aniline and forms a diazo salt, generally diazonium chloride is formed as chlorine ion is already present in the solution because of the hydrochloric acid added. The diazonium salt thus formed when reacted with 2- naphthol in alkaline conditions gives an orange or red coloured dye.
However in case of aliphatic amines, the amine undergoes substitution reaction and an alcohol is formed along with the release of nitrogen gas and an effervescence is observed.
Thus, we can use azo dye to distinguish between aliphatic amines and aniline.

Hence, option A is correct.

Note:
Azo dyes are a variety of dyes containing a nitrogen- nitrogen double bond. They are synthetic dyes used to impart colour to leather, textiles and food products. Azo dyes have strong, distinct colors, low on cost and are easy to apply.