Chemistry · Amines · NEET
Diazotisation is the reaction that turns a primary aromatic amine into a diazonium salt. You add sodium nitrite (NaNO2) and a mineral acid like HCl to the amine. The NaNO2 + HCl make nitrous acid (HNO2) in the flask. The HNO2 then reacts with the -NH2 group to build the -N2+ group. Example: C6H5NH2 + NaNO2 + 2HCl -> C6H5N2+Cl- + NaCl + 2H2O. You must do it at 273-278 K (0-5 degrees C).
Two reasons. First, nitrous acid (HNO2) is itself unstable and only stays in solution when cold, so you keep the mix in an ice bath. Second, the benzenediazonium salt that forms is only stable at low temperature. If the flask warms above 278 K, the salt decomposes: it reacts with water to give phenol and releases nitrogen gas (C6H5N2+Cl- + H2O -> C6H5OH + N2 + HCl). So cold keeps both the reagent and the product alive.
In benzenediazonium ion, the positive -N2+ group is attached to the benzene ring. The ring's electrons (its pi cloud) spread the positive charge over the whole ring by resonance. This resonance stabilisation lets the salt survive when kept cold. In an aliphatic diazonium salt (like C2H5N2+), the alkyl group has no pi electrons to share the charge, so nothing stabilises it. It falls apart the moment it forms, throwing out N2 gas. This exact idea was tested in NEET 2022 and 2026.
It is so unstable it never really exists as a salt. When a primary aliphatic amine meets HNO2, the R-N2+ ion forms and instantly loses N2 gas. The leftover carbon then grabs an OH from water and you get an alcohol. Example: C2H5NH2 + HNO2 -> C2H5OH + N2 (up) + H2O. So the useful sign of a primary aliphatic amine with HNO2 is brisk bubbling of nitrogen gas. NEET 2026 keyed the product of this exact route as ethanol.
Even though it is stable in cold solution, the dry solid is dangerous. When dry, benzenediazonium chloride can decompose suddenly and even explode. That is why we prepare it fresh in solution and use it straight away in the same flask for the next reaction (like Sandmeyer or azo coupling). NEET 2025 stated 'it decomposes easily in the dry state' - that statement is correct.
Only primary amines give diazonium salts. A primary aromatic amine (like aniline) gives a stable-when-cold diazonium salt. A secondary amine gives an N-nitrosamine (a yellow oily compound), and a tertiary amine gives a different product (a nitroso salt or ring nitrosation). So the diazotisation test really works for primary amines, and cleanly for primary aromatic amines.
Statement I: Primary aliphatic amines react with HNO2 to give unstable diazonium salts. Statement II: Primary aromatic amines react with HNO2 to form diazonium salts which are stable even above 300 K. Choose the most appropriate answer.
The major product Z formed in the following sequence of reactions is: C2H6 ->[Cl2/UV] X ->[NH3] Y ->[(i) NaNO2/HCl (ii) H2O] Z
Statement I: Benzenediazonium salt is prepared by the reaction of aniline with acid at 273-278 K. It decomposes easily in the dry state. Statement II: Insertion of iodine into the benzene ring is difficult, and hence iodobenzene is prepared through the reaction of the benzenediazonium salt with KI. Choose the most appropriate answer.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Sodium nitrite (NaNO2) plus a mineral acid, usually HCl. Together they make nitrous acid (HNO2) in the flask, which is the actual reacting species. The mix is written as NaNO2/HCl at 273-278 K.
Primary aromatic amines like aniline. They give arenediazonium salts that are stable in cold solution and can be used in further reactions. Primary aliphatic amines also react, but their diazonium salts fall apart instantly into alcohol plus N2.
C6H5N2+Cl-. It has a benzene ring joined to -N2+ (two nitrogens with a positive charge), balanced by a chloride ion.
Because both the nitrous acid reagent and the benzenediazonium salt product are unstable when warm. The cold 273-278 K bath keeps them from decomposing before you can use the salt.
Steady bubbling of nitrogen gas. The unstable aliphatic diazonium ion loses N2 at once and forms an alcohol, so you see brisk effervescence of N2.
