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Slide 1

Azo Compounds

Azo Compounds

Slide 2

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.


Azo group

Slide 3

Diazonium salts

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.


Prepare fresh and use immediately.

Slide 4

How the salt is made.

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.


benzenediazonium ion

Slide 5

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.

Slide 6

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.

Slide 7

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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