Chemistry · Amines · NEET
It is electrophilic aromatic substitution. NCERT states clearly that the reaction of the diazonium salt with aniline (or phenol) is an example of electrophilic substitution. The diazonium cation C6H5-N2+ acts as a weak electrophile and attacks the electron-rich ring of phenol or aniline. Do not confuse it with the OTHER diazonium reactions (Sandmeyer, phenol formation, iodination) where the diazonium group is replaced - those lose N2. In coupling, nothing is replaced on the diazonium; the two rings simply join.
The -OH group of phenol and the -NH2 group of aniline are strong ortho/para directors that push electron density into the ring. The diazonium cation is a bulky, weak electrophile, so it prefers the less crowded para position. NCERT specifically says phenol is coupled at its para position to form p-hydroxyazobenzene, and aniline gives p-aminoazobenzene. If the para position is blocked, coupling can go ortho, but for NEET the standard answer is para.
No. This is the key point. Coupling is the ONLY major diazonium reaction that keeps both nitrogen atoms - the -N=N- azo bridge in the product comes from those very nitrogens. Reactions like Sandmeyer, Balz-Schiemann, deamination and phenol formation all replace the -N2+ group and evolve N2 gas. So NCERT groups reactions into two families: replacement of the diazo group (N2 lost) and retention of the diazo group (coupling, N2 kept).
The product has both aromatic rings joined through the -N=N- bond, giving one long conjugated system (extended delocalisation of electrons). This large conjugation lets the molecule absorb visible light, so it appears coloured. NCERT says these compounds are often coloured and are used as dyes. p-hydroxyazobenzene is an orange azo dye. More conjugation shifts absorption toward longer wavelengths, deepening the colour.
The benzenediazonium ion C6H5-N2+ is the electrophile (electron-poor, weak). Phenol or aniline is the substrate (electron-rich). The electrophile is weak, so it can only attack a strongly activated ring - plain benzene or benzene bearing deactivating groups will NOT couple. That is why coupling needs an activated partner like -OH or -NH2.
A given nitrogen-containing aromatic compound A reacts with Sn/HCl, followed by HNO2, to give an unstable compound B. B, on treatment with phenol, forms a beautiful coloured compound C with molecular formula C12H10N2O. The structure of compound A is
Try the real previous-year questions from this chapter — each with the answer and a full solution.
p-Hydroxyazobenzene (an orange azo dye), C6H5-N=N-C6H4-OH. The phenol couples at its para position. Its molecular formula is C12H10N2O.
p-Aminoazobenzene, C6H5-N=N-C6H4-NH2. Aniline also couples at the para position through its activated ring.
Coupling with phenol is usually done in mildly alkaline medium and coupling with aromatic amines in mildly acidic medium, at low temperature. The diazonium salt itself is kept cold (273-278 K) because it is unstable when warm.
The diazonium ion is a very weak electrophile, so it needs a strongly activated ring. Benzene has no activating group, so it will not couple. Only rings carrying strong electron-donors like -OH or -NH2 react.
It is a retention reaction. Both nitrogens stay in the product as the -N=N- azo bond, and no N2 gas is evolved. This distinguishes it from replacement reactions such as Sandmeyer or phenol formation.
