Diazotization

Diazotization is the conversion of a primary aromatic amine into a diazonium salt using nitrous acid generated in situ from sodium nitrite and a mineral acid. The benzenediazonium chloride produced is thermally unstable: above about 278 K it readily decomposes, reacting with water to give phenol and liberating nitrogen gas. Maintaining the temperature between 273 and 278 K keeps the diazonium ion intact long enough to be used in subsequent substitution or coupling reactions. The option that aniline freezes is incorrect because the freezing concern is about the product's stability, not the amine. Sodium nitrite is not destabilised at low temperature, so that option is false. The reaction does not require heat to initiate; in fact warming is harmful, so the endothermic-start claim is wrong. This precise low-temperature requirement is a standard NCERT and JEE detail because diazonium chemistry is central to aromatic synthesis, where the cold salt is later used for Sandmeyer, coupling, or hydrolysis steps. As a plausibility check, the observed evolution of nitrogen gas and formation of phenol whenever the mixture warms confirms that decomposition is the controlling hazard the cooling prevents, and that the ice-bath temperature window is a kinetic safeguard rather than a thermodynamic one.