Organic Compounds Containing Nitrogen - Diazonium salts: Preparation and chemical reactions

Diazonium salts are organic compounds with the general formula $ArN_2^+X^-$, where $Ar$ is an aryl group and $X^-$ represents anions like $Cl^-$, $Br^-$, $HSO_4^-$, or $BF_4^-$.

Preparation (Diazotization): Primary aromatic amines react with nitrous acid ($HNO_2$), produced in situ from $NaNO_2$ and $HCl$, at low temperatures ($273 - 278$ $K$). The reaction is $C_6H_5NH_2 + NaNO_2 + 2HCl \xrightarrow{273-278 K} C_6H_5N_2^+Cl^- + NaCl + 2H_2O$.

Stability: Benzenediazonium salts are stable for a short time in cold aqueous solution due to the resonance stabilization of the $N_2^+$ group with the benzene ring.

Sandmeyer Reaction: Replacement of the diazo group by $Cl$, $Br$, or $CN$ using cuprous salts ($Cu_2Cl_2$, $Cu_2Br_2$, or $CuCN$).

Gattermann Reaction: A modification of the Sandmeyer reaction where copper powder and the corresponding halogen acid ($HCl$ or $HBr$) are used instead of cuprous salts.

Balz-Schiemann Reaction: Introduction of a fluorine atom by treating the diazonium salt with fluoroboric acid ($HBF_4$) to form $ArN_2^+BF_4^-$, followed by heating to produce $ArF$.

Replacement by Hydroxyl group: Warming the diazonium salt solution to $283$ $K$ or above results in hydrolysis to form phenols: $C_6H_5N_2^+Cl^- + H_2O \rightarrow C_6H_5OH + N_2 + HCl$.

Coupling Reactions: Diazonium salts react with phenols or aromatic amines to form highly colored azo compounds ($Ar-N=N-Ar'$). This is an electrophilic aromatic substitution reaction.