Q.Complete the following reactions :
• $Ar-NH_2 \xrightarrow{?} Ar-N_2^{\oplus}Cl^{\ominus} \xrightarrow[Cupprous\ chloride]{HCl} ?$
• $Ar-N_2^{\oplus}Cl^{\ominus} \xrightarrow{H_3PO_2} ?$

Aromatic primary amines react with nitrous acid (from NaNO2 + HCl, at 273-278 K) to give arene diazonium salts, resonance-stabilised and reasonably stable at low temperature, making them versatile synthetic intermediates. The diazonium group (-N2+) is a very good leaving group, so these salts undergo nucleophilic substitution with a wide range of nucleophiles, DISPLACING the diazo group as N2 gas: the Sandmeyer reaction (CuCl/HCl, CuBr/HBr or CuCN/KCN, giving Ar-Cl, Ar-Br or Ar-CN respectively, good yields), the Gatterman reaction (Cu powder + HCl or HBr, giving Ar-Cl or Ar-Br), iodoarene formation (KI, giving Ar-I) …