Q.Aniline is treated with $\text{NaNO}_2$ and dilute $\text{HCl}$ at $0$–$5^\circ\text{C}$ (diazotization). Write the equation for the formation of benzenediazonium chloride and explain why the low temperature is essential.

$\text{NaNO}_2$ and $\text{HCl}$ generate nitrous acid in situ, which diazotizes aniline's free $-\text{NH}_2$ group to the diazonium salt: $\text{C}_6\text{H}_5\text{NH}_2 + \text{NaNO}_2 + 2\text{HCl} \xrightarrow{0\text{–}5^\circ\text{C}} \text{C}_6\text{H}_5\text{N}_2^+\ \text{Cl}^- + \text{NaCl} + 2\text{H}_2\text{O}$. Even though the aromatic diazonium ion is stabilised relative to an aliphatic one by resonance delocalisation of its charge into the ring, it is still a comparatively reactive species that decomposes (largely by hydrolysis to phenol, with loss of $\text{N}_2$) if the temperature …

Write the diazotization equation, then justify the temperature by the sa …

Believing the low temperature is needed to slow the diazotization reaction itself -- it is the PRODUCT's instability on warming t …