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Azo Compounds. What are azo compounds?. Contain the -N=N- group. Where R and R’ are arene groups more stable than alkyl groups. Azo group is stabilised by becoming part of extended delocalised system. Result of a coupling reaction between a diazonium salt and a coupling agent. R-N=N-R'.

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what are azo compounds
What are azo compounds?
  • Contain the -N=N- group.
  • Where R and R’ are arene groups more stable than alkyl groups.
  • Azo group is stabilised by becoming part of extended delocalised system.
  • Result of a coupling reaction between a diazonium salt and a coupling agent.

R-N=N-R'

Azo group

diazonium salts

N+N Cl-

Diazonium salts
  • Only stable salts are aromatic - not particularly stable.
  • Lose -N+N as N2(g)
  • Electron rich benzene ring stabilises the -N+N group but decomposition occurs above about 5oC.
  • Add cold soln. sodium nitrite (NaNO2) to arylamine soln. In dilute acid below 5oC.
  • Diazotisation.
  • Prepare fresh and use immediately.
how the salt is made

+ HNO2 + H+ 

N+N + 2 H2O

NH2

How the salt is made.
  • Acid reacts with sodium nitrite to form unstable nitrous acid.
  • NaNO2 (aq) + HCl (aq)HNO2(aq) + NaCl (aq)
  • Nitrous acid reacts with the arylamine.

phenylamine

benzenediazonium ion

diazo coupling reactions
Diazo coupling reactions
  • A diazonium salt reacts with another compound containing a benzene ring called a coupling agent.
  • Diazonium salt acts as an electrophile - reacts with benzene ring of coupling agent.
  • Coloured precipitate of azo compound immediately forms.
  • Important use as dyes.
coupling with phenols
Coupling with phenols
  • Benzenediazonium salt and alkaline phenol gives a yellow orange azo compound
  • Benzenediazonium salt and alkaline naphthalen-2-ol gives a red azo compound.
coupling with amines
Coupling with amines
  • Diazonium salts couple with arylamines.
  • Benzenediazonium salt and phenylamine gives a yellow azo compound.
  • Use different diazonium salts and coupling agents to make different colours.
  • Azo compounds are stable so dyes do not fade.
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