Diazonium Salts 🎯

Diazonium salts have the general formula \(\mathrm{R–N_2^{+}X^{-}}\), where R is an aryl group and X− can be Cl−, Br−, HSO4−, BF4−, etc. :contentReference[oaicite:0]{index=0}

  • The \(\mathrm{N_2^{+}}\) part is called the diazonium group. :contentReference[oaicite:1]{index=1}
  • Names are built by adding “diazonium” to the parent hydrocarbon name plus the anion name.
     • Example 1: \(\mathrm{C_6H_5N_2^{+}Cl^{-}}\) → benzenediazonium chloride
     • Example 2: \(\mathrm{C_6H_5N_2^{+}HSO_4^{-}}\) → benzenediazonium hydrogensulphate :contentReference[oaicite:2]{index=2}

Stability 😊

  • Primary aliphatic amines give very unstable alkyldiazonium salts. :contentReference[oaicite:3]{index=3}
  • Primary aromatic amines form arenediazonium salts that survive briefly in solution if kept cold (273 – 278 K). :contentReference[oaicite:4]{index=4}
  • The short-lived stability of arenediazonium ions comes from resonance delocalisation of the positive charge. :contentReference[oaicite:5]{index=5}

Method of Preparation 🧪

The process of converting a primary aromatic amine into a diazonium salt is called diazotisation. Benzenediazonium chloride is made by treating aniline with nitrous acid at 273 – 278 K. Nitrous acid is generated in situ from sodium nitrite and hydrochloric acid. :contentReference[oaicite:6]{index=6}

\[ \mathrm{C_6H_5NH_2 + NaNO_2 + 2HCl \xrightarrow{273\text{–}278\,K} C_6H_5N_2^{+}Cl^{-} + 2H_2O + NaCl} \]

Because the salt decomposes quickly, it is used immediately after it forms—no long-term storage! ⌛ :contentReference[oaicite:7]{index=7}

Why keep it cold? 🌡️

Low temperature slows down unwanted side reactions that would destroy the diazonium ion, giving you enough time to carry out further transformations.

Quick Recap for NEET 🚀

  1. Remember the generic structure \(\mathrm{R–N_2^{+}X^{-}}\) and that it comes mainly from aryl (not alkyl) amines.
  2. Diazotisation needs an ice-cold (\(\approx 273\; \text{K}\)) acidic solution of NaNO2 + HCl.
  3. Benzenediazonium chloride is your prototype reagent—know its preparation and the balanced equation.
  4. Stability is resonance-driven; alkyldiazonium salts are too unstable for practical work.
  5. “Use it fresh!”—diazonium salts decompose rapidly, so plan the next step right away.