Read the passage carefully and answer.
Organic compounds derived from \( NH_3 \) by replacing one or more H-atoms with alkyl or aryl groups will form primary, secondary and tertiary amines. The common methods which include their preparations are reduction of nitro group, amide group and nitrile group, Gabriel phthalimide synthesis and ammonolysis of alkyl halides. Amines are basic in nature due to the presence of a lone pair of electrons on the nitrogen atom. Important chemical properties of amines include acylation, alkylation and their interaction with nitrous acid. There are various tests which can distinguish different types of amines from each other, for e.g. carbylamine test, Hinsberg's test, coupling reaction etc. Another important class of compounds which can be derived from primary amines is azo compounds. These compounds are intensely colored and widely used as dyes in textile, leather and food industries.
Correctly predict the final product and the type of reaction for the following reaction:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → p-Hydroxyazobenzene, Electrophilic Substitution Reaction.
In a coupling reaction the diazonium group is kept, and the diazonium ion joins another aromatic ring through an –N=N– bridge. The diazonium ion, \( \ce{C6H5N2^+} \), is a weak electrophile, so it attacks only a strongly activated ring such as phenol (in alkaline medium, as the phenoxide ion). The attack takes place at the para position of phenol.
\( \displaystyle \ce{C6H5N2^+Cl^- + C6H5OH} \xrightarrow{\ce{OH^-}} p\text{-}\ce{C6H5-N=N-C6H4OH + Cl^- + H2O} \)
The product, p-hydroxyazobenzene, is an orange azo dye. Since an electrophile replaces a hydrogen atom of the phenol ring, this is an electrophilic substitution reaction.
Why the other options are incorrect
Option (1) p-Aminoazobenzene, Nucleophilic Substitution Reaction. — Incorrect. p-Aminoazobenzene is the product of coupling with aniline, not phenol, and the reaction is not nucleophilic.
Option (2) p-Aminoazobenzene, Electrophilic Substitution Reaction. — Incorrect. The type is right, but the product needs aniline as the coupling partner; with phenol the ring carries –OH.
Option (3) p-Hydroxyazobenzene, Nucleophilic Substitution Reaction. — Incorrect. The product is right, but the diazonium ion is the electrophile that attacks the electron-rich ring, so the substitution is electrophilic.
NCERT (2026-27 edition):
Reactions Involving Retention of Diazo Group
