The azo compounds formed in coupling reactions are often coloured and are used as dyes. Which of the following describes the structure of these azo compounds?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → An extended conjugate system having both the aromatic rings joined through the –N=N– bond
Coupling belongs to the reactions in which the diazo group is retained.
The diazonium ion attacks the phenol or aniline ring at its para position (electrophilic substitution), and the two rings end up joined by an –N=N– bridge. The double bonds of both rings and of –N=N– form one long conjugated system.
For example, coupling with phenol gives p-hydroxyazobenzene (an orange dye):
\( \displaystyle \ce{C6H5N2^+Cl^- + C6H5OH} \xrightarrow{\ce{OH-}} \ce{C6H5-N=N-C6H4-OH + Cl^- + H2O} \)
Why the other options are incorrect
Option (1) The –N2+ group remains attached to one aromatic ring as a positively charged group — Incorrect. The diazo nitrogen atoms stay, but the diazonium ion has coupled with the second ring: they now form the neutral –N=N– link, not an –N2+ group.
Option (2) The two aromatic rings are joined directly by a carbon–carbon bond, and the nitrogen is lost as N2 gas — Incorrect. Loss of nitrogen happens in the other class of reactions (displacement of nitrogen, such as the Sandmeyer reaction). In coupling the nitrogen is retained.
Option (4) The –N=N– bond is joined to the second ring at the meta position to its –OH or –NH2 group — Incorrect. Phenol is coupled at its para position, and aniline gives p-aminoazobenzene.
NCERT (2026-27 edition):
9.9 Chemical Reactions – “The azo products obtained have an extended conjugate system having both the aromatic rings joined through the –N=N– bond. These compounds are often coloured and are used as dyes.”