Azo Compounds (Cambridge (CIE) A Level Chemistry): Revision Note

Azo Compounds

• Azo (or diazonium) compounds are organic compounds that have an R₁-N=N-R₂ group

• They are often used as dyes and are formed in a coupling reaction between the diazonium ion and an alkaline solution of phenol

Example compound containing an azo group

Azo compounds are characterized by the presence of an R₁-N=N-R₂ group

Coupling of benzenediazonium chloride with phenol in NaOH

• Azo compounds can be formed from the coupling reaction of a benzenediazonium chloride salt with alkaline phenol

• Making an azo dye is a multi-step process:

  • Step 1 - Formation of nitrous acid

    • The nitrous acid, HNO₂, is so unstable that it needs to be prepared in a test-tube by reacting sodium nitrite (NaNO₂) and dilute hydrochloric acid (HCl) while keeping the temperature below 10 ^oC using ice

Forming nitrous acid, HNO₂

Nitrous acid can be prepared in a test-tube by reacting sodium nitrate with dilute hydrochloric acid

• Step 2 - Diazotisation

  • This is the reaction between nitrous acid and phenylamine to form a diazonium ion

  • Dilute acid is used for this step, e.g. HCl

  • The reaction mixture must be kept < 10 ^oC using ice to prevent the diazonium ion from thermally decomposing to benzene and nitrogen

Forming benzenediazonium chloride 

Benzenediazonium chloride is an unstable diazonium salt

• Step 3 - Coupling reaction

  • The diazonium ion acts as an electrophile and substitutes into the benzene ring of the phenol, at the 4th position

  • Alkaline conditions are required to deprotonate the organic product and form the azo compound

Forming the azo compound

The azo compound is formed by the electrophilic substitution reaction of benzenediazonium chloride and phenol

• The delocalised electrons in the π bonding systems of the two benzene rings are extended through the -N=N- which acts as a bridge between the two rings

• As a result of the delocalisation of electrons throughout the compound, azo compounds are very stable

Making other azo dyes

• Other dyes can be formed via a similar route as described above

• For example, the yellow dye can be formed from the coupling reaction between benzenediazonium chloride and C₆H₅N(CH₃)₂ instead of phenol (C₆H₅OH)

 Making yellow azo dye

The yellow azo dye is formed via a coupling reaction between benzenediazonium chloride and C₆H₅N(CH₃)₂