Diazotisation Reaction — the NEET Chemistry reaction: mechanism, reagents, conditions, structures and exam traps.
Diazotisation Reaction Diazotisation is the reaction of a primary aromatic amine with nitrous acid (generated in situ from sodium nitrite and a strong mineral acid like HCl or H2SO4) at a low temperature (0-5 °C) to form a highly reactive and unstable arenediazonium salt. This reaction is crucial for synthesizing various aromatic compounds through subsequent reactions of the diazonium salt. Initially, a clear solution of the primary aromatic amine and acid is mixed with sodium nitrite. Maintaining 0-5 °C is crucial, often by using an ice bath. If the temperature is allowed to rise, effervescence (evolution of colorless nitrogen gas) will be observed due to the decomposition of the diazonium salt. The resulting arenediazonium salt solution is typically colorless, but it's highly reactive and often immediately used for coupling reactions or other transformations. The reaction is exothermic, which necessitates strict temperature control (0-5 °C) using an ice bath to prevent premature decomposition of the unstable diazonium salt. Generation of nitrous acid: NaNO2 + HCl → HNO2 + NaCl Protonation of nitrous acid: HNO2 + H+ → H2O+-NO Formation of nitrosonium ion: H2O+-NO → H2O + NO+ (the active electrophile) Electrophilic attack: Ar-NH2 + NO+ → Ar-NH2+-NO Deprotonation: Ar-NH2+-NO → Ar-NH-NO + H+ Tautomerization (nitrosamine to diazoic acid): Ar-NH-NO → Ar-N=N-OH Protonation of oxygen: Ar-N=N-OH + H+ → Ar-N=N-OH2+ Loss of water to form diazonium ion: Ar-N=N-OH2+ → Ar-N+=N + H2O Failing to maintain the temperature between 0-5 °C. Higher temperatures lead to rapid decomposition of the diazonium salt to a phenol and nitrogen gas. Attempting diazotisation on primary aliphatic amines. These form highly unstable diazonium salts that immediately decompose to carbocations, leading to rearrangements, eliminations, and substitutions, not stable diazonium salts. Forgetting to use an acid. The acid is essential for generating nitrous acid from sodium nitrite and subsequently forming the highly electrophilic nitrosonium ion (NO+). Confusing the diazonium salt as a final stable product. It's usually an unstable intermediate used for further synthetic transformations.