Aniline is an organic compound with the formula C6H5NH2. Consisting of a phenyl group attached to an amino group, aniline is the prototypical aromatic amine. Its main use is in the manufacture of precursors to polyurethane and other industrial chemicals. Like most volatile amines, it has the odor of rotten fish. It ignites readily, burning with a smoky flame characteristic of aromatic compounds.
Other uses include rubber processing chemicals (9%), herbicides (2%), and dyes and pigments (2%). As additives to rubber, aniline derivatives such as phenylenediamines and diphenylamine, are antioxidants. Illustrative of the drugs prepared from aniline is paracetamol (acetaminophen, Tylenol).

Dye tests are used to identify compounds that have the ability to form colored complexes with certain reagents. Aniline is a compound that gives a positive dye test, while the other options (methylamine, diphenylamine, and ethylamine) do not.

Aniline (option A) is an aromatic amine with the chemical formula C6H5NH2. It contains a benzene ring connected to an amino group (-NH2). This amino group is responsible for the ability of aniline to give a dye test.

Here is a detailed explanation of why aniline gives a positive dye test:

1. Aromaticity of Aniline:
Aniline is an aromatic compound due to the presence of a benzene ring. Aromatic compounds have a stable, delocalized π electron system. The presence of this electron-rich aromatic system in aniline allows it to undergo various reactions, including dye tests.

2. Dye Test Principle:
Dye tests are based on the formation of colored complexes between a compound and a specific reagent. In the case of aniline, it reacts with diazonium salts to form colored azo dyes.

3. Reaction with Diazonium Salts:
Diazonium salts (e.g., benzenediazonium chloride) are formed by the reaction of primary aromatic amines (such as aniline) with nitrous acid (HNO2). Diazonium salts are highly reactive and can undergo various reactions, including coupling reactions with other compounds.

4. Coupling Reaction:
When aniline reacts with a diazonium salt, it undergoes a coupling reaction in which the amino group (-NH2) of aniline is replaced by the diazonium group (-N2+). This reaction forms an azo compound, which is a colored compound.

5. Formation of Azo Dyes:
The azo compound formed from the coupling reaction of aniline and a diazonium salt is a colored compound. The color of the azo compound depends on the nature of the diazonium salt used.

6. Observation of Dye Test:
When aniline is subjected to a dye test, it forms a colored solution or precipitate upon reaction with a diazonium salt. This color change indicates the presence of aniline, confirming the positive dye test result.

In summary, aniline gives a positive dye test because it contains an amino group that can react with diazonium salts to form colored azo compounds. The formation of these colored compounds allows for the easy identification of aniline in a dye test.

Aniline give azo.dye test