When an primary aromatic amine is treated with `NaNO_(2)+HCI` at `0^(@)-5^(@)C`, a diazonium salt is formed and the reaction is called diazo reaction. In this reaction mineral acid must be added to prevent the coulping reaction of diazonium salt with excess of aryl amine. diazonium salt is highly in the synthesis of number of coloured dyes.
When 2,4-dinitrophenol react with `NaNO_(2)+HCI` at `5^(@)C` followed by reaction with anisole, a coloured compound is formed which can be given as:

Explanation

(i) Dyes containing -N=N-as the functional group are called azo dyes. For example, the simplest azo dye is aniline yellow obtained by coupling benzenediazonium chloride with aniline.

(ii) An example of synthetic azo dye which has been used to colour food is tetrazine. it is considered to be a cancer suspect.