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Amines - Method of Preparation of Diazonium Salts

Grade 12CBSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Diazotization is the process of converting primary aromatic amines into diazonium salts by treating them with nitrous acid (HNO2HNO_2) at low temperatures (273–278 K273\text{--}278\ K).

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Nitrous acid is highly unstable and is therefore produced in situ by the reaction of sodium nitrite (NaNO2NaNO_2) with a mineral acid like HClHCl or H2SO4H_2SO_4.

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The reaction must be carried out at 0∘C0^\circ C to 5∘C5^\circ C. If the temperature rises, the diazonium salt decomposes, reacting with water to form phenol and evolving N2N_2 gas.

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Aromatic diazonium salts are relatively stable due to the dispersal of the positive charge over the benzene ring through resonance.

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Primary aliphatic amines react with nitrous acid to form highly unstable aliphatic diazonium salts, which decompose immediately to yield a mixture of alcohols, alkenes, and alkyl halides, with the evolution of N2N_2 gas.

📐Formulae

C6H5NH2+NaNO2+2HCl→273−278 KC6H5N2+Cl−+NaCl+2H2OC_6H_5NH_2 + NaNO_2 + 2HCl \xrightarrow{273-278\ K} C_6H_5N_2^+Cl^- + NaCl + 2H_2O

NaNO2+HCl→HNO2+NaClNaNO_2 + HCl \rightarrow HNO_2 + NaCl

[C6H5N≡N]+Cl− (Benzenediazonium chloride)[C_6H_5N \equiv N]^+Cl^- \text{ (Benzenediazonium chloride)}

💡Examples

Problem 1:

Explain why aniline does not form a stable diazonium salt at room temperature.

Solution:

At room temperature (above 278 K278\ K), the benzenediazonium salt formed is thermally unstable. It reacts with the water present in the reaction mixture via a nucleophilic substitution reaction: C6H5N2+Cl−+H2O→C6H5OH+N2+HClC_6H_5N_2^+Cl^- + H_2O \rightarrow C_6H_5OH + N_2 + HCl.

Explanation:

The diazonium group (−N2+-N_2^+) is an excellent leaving group. The kinetic energy at room temperature is sufficient to break the C−NC-N bond, leading to the formation of phenol and the release of nitrogen gas.

Problem 2:

How is nitrous acid prepared during the diazotization reaction?

Solution:

Nitrous acid (HNO2HNO_2) is prepared in the reaction flask (in situ) by reacting sodium nitrite (NaNO2NaNO_2) with cold dilute hydrochloric acid (HClHCl): NaNO2+HCl→HNO2+NaClNaNO_2 + HCl \rightarrow HNO_2 + NaCl.

Explanation:

Since HNO2HNO_2 is an unstable weak acid that decomposes easily, it cannot be stored and must be generated freshly at the time of the reaction.