CHEMISTRY CLASS XII

UNIT 13

ORGANIC COMPOUNDS CONTAINING NITROGEN

PART II

DIAZONIUM SALTS

Prepared by

Mr, Sunil Urade, PGT Chemistry JNV Chandrapur

Mr. G.T. Hirve, PGT Chemistry JNV Latur

Mr.Ajit Sharma, PGT Chemistry JNV Gandhinagar

REGIONAL OFFICE, PUNE

OUTLINE

• Review of previous session
• Introduction:What are the diazonium salts?
• Structure
• Method of Preparation of benzenediazonium chloride
• Physical Properties of benzenediazonium chloride
• Chemical properties of benzenediazonium chloride
• Importance

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DIAZONIUM SALTS

• The diazonium salt have the general formula RN₂⁺X^-

where R stands for an aryl group and X^- may be Cl^- ,Br^-, HSO₄^- BF₄^- etc.

• They are named by suffixing diazonium to the name of the parent hydrocarbon from which they are formed
• The N₂⁺ group is called diazonium group.
• For example, C₆H₅N₂⁺Cl^- benzenediazoniumchloride

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Stability of benzene diazonium salt

• The stability of benzenediazonium ion is due to the resonance effect

Preparation (Diazotization)

• Benzenediazonium chloride is prepared by the reaction of aniline with nitrous acid at 273-278K.
• Nitrous acid is produced in the reaction mixture by the reaction of sodium nitrite with hydrochloric acid.
• The conversion of primary aromatic amines into diazonium salts is known as diazotisation.

Physical Properties

• Benzenediazonium chloride is a colourless crystalline solid.

• It is readily soluble in water and is stable in cold but reacts with water when warmed

• It decomposes easily in the dry state

• Benzenediazonium fluoroborate is water insoluble and stable at room temperature.

Chemical Reactions of Diazonium salt

Reactions involving displacement of Nitrogen

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• Diazonium group being a very good leaving group, is substituted by other groups such as Cl^–, Br^–, I^–, CN^– and OH^– which displace nitrogen from the aromatic ring.

• The nitrogen formed escapes from the reaction mixture as a gas.

1. Replacement by halide or cyanide ion:

Sandmeyer reaction : The Cl^–, Br^– and CN^– nucleophiles introduced in the benzene ring in the presence of Cuprous ion.

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Gatterman reaction :

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2. Replacement by iodide ion:

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3. Replacement by fluoride ion:

4. Replacement by H:

Certain mild reducing agents like phosphinic acid or ethanol reduce diazonium salts to arenes and themselves get oxidised to phosphorous acid and ethanal, respectively.

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5. Replacement by hydroxyl group:

6. Replacement by –NO2 group:

When diazonium fluoroborate is heated with aqueous sodium nitrite solution in the presence of copper, the diazonium group is replaced by –NO2 group.

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Coupling reactions

Importance of Diazonium Salts in Synthesis of aromatic Compounds

• Aryl fluorides and iodides cannot be prepared by direct halogenation.
• The cyano group cannot be introduced by nucleophilic substitution of chlorine in chlorobenzene but cyanobenzene can be easily obtained from diazonium salt.
• The replacement of diazo group by other groups is helpful in preparing those substituted aromatic compounds which cannot be prepared by direct substitution in benzene or substituted benzene.

Summary

R O PUNE

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• Preparation of diazonium salt

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• Important reactions of diazonium salts

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• The importance of diazonium salts in organic synthesis.

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Home work

1.Writes short notes on the following

(i) Coupling reaction

(ii) Diazotisation

2 How will you convert

(i)Benzene diazonium chloride in to p-hydroxyazo benzene

(ii)Benzene diazonium chloride in to p- aminoazo benzene

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Thanks