Q.What is diazotisation ? Write diazotisation reaction of aniline.

Step 1. Define diazotisation (section 13.6.6b). Diazotisation is the reaction of an AROMATIC primary amine with nitrous acid, carried out at low temperature (273-278 K), converting the amine into a reasonably stable arene diazonium salt. As with all nitrous-acid chemistry in this chapter, the nitrous acid itself is generated in situ from aqueous sodium nitrite plus hydrochloric acid, since it cannot be stored.

Step 2. Explain why a low temperature is essential. Although aromatic diazonium salts are markedly more stable than their aliphatic counterparts (which decompose essentially immediately, section 13.6.6a), they are still only stable within a narrow low-temperature window; above roughly 278 K they too begin to decompose, so the whole reaction and any subsequent handling of the diazonium salt is kept close to 0 degrees C.

Step 3. Write aniline's own diazotisation reaction. C6H5-NH2 + NaNO2 + 2 HCl, at 273-278 K, gives C6H5-N2(+)Cl(-) (benzenediazonium chloride) + NaCl + 2 H2O.

Defining diazotisation from section 13.6.6(b) and writing its specific worked equation for aniline, including the essential low-temperature condition

• Omitting the essential low-temperature condition (273-278 K), which is not a minor detail but the very reason an AROMATIC diazonium salt can be isolated/used at all, unlike the aliphatic case where the analogous salt decomposes regardless of temperature control.
• Confusing diazotisation (forming the diazonium salt) with azo coupling (section 13.7.2, a SEPARATE, subsequent reaction of that already-formed diazonium salt with a second aromatic compound).