Question

Azo dye is prepared by the coupling of phenol and?
A. Diazonium chloride
B. o-nitroaniline
C. Benzoic acid
D. Chlorobenzene

Answer 1

Hint: Azo dye is an organic compound formed by a diazo coupling reaction. Azo dye has an azo functional group i.e., where this azo bond is generally attached to the aromatic rings. This reaction is an electrophilic substitution reaction where phenol act as a nucleophile.

Complete Step by Step Solution:
From the option, we need to find the one which is electrophilic so that our nucleophile i.e., phenol can attach to it and also can form an azo bond to prepare the azo dye. In the option, only diazonium chloride is the compound with a diazo bond i.e., and can directly form an azo bond with nucleophiles.

Image: Azo Dye preparation by coupling of phenol and diazonium chloride salt

So, option A is correct.

Additional Information: Options B, C and D cannot form azo dye because first, they cannot make an azo bond and second, they cannot attract nucleophiles better than diazonium chloride. Because in the o-nitroaniline nitro functional group is present which is an electron-withdrawing character and reduces the nucleophilic character of an aromatic ring. In benzoic acid again, the carboxylic acid functional group is taking part in the delocalisation of electrons of benzene and hence reduces the nucleophilic properties. In chlorobenzene -I effect of chlorine decreases the electron density on the benzene ring and reduces its nucleophilic character. Azo dyes are widely used in the colouring of paper, textiles and printing industries.

Note: Azo bond is \[ - N = N - \] and diazo bond is \[ - \mathop N\limits^ \oplus \equiv N\]. Electrophiles are those which have more electron density and attack nucleophiles to give a nucleophilic reaction and nucleophiles have less electron density and attach with electrophiles to give an electrophilic reaction.