Azo Compounds

Azo compounds contain the –N=N– group:
In aromatic azo compounds, the R groups are arene rings; the structures of these are more stable than if the R groups are alkyl groups.
This is because the –N=N– group becomes part of an extended delocalised system involving the arene groups.
The aromatic azo groups are highly coloured and are often used as dyes.
Aromatic azo compounds are formed by a coupling reaction between a diazonium salt and a coupling agent.

Diazonium Salts

The diazonium salts are very unstable; the only relatively stable diazonium salts are the aromatic ones, and these are not particularly stable.
This is because the presence of the benzene ring with its high electron density stabilises the –⁺ group.
Benzenediazonium chloride is an example of a diazonium salt:
In aqueous solution, Benzenediazonium chloride decomposes above temperatures of 5^oC and the solid compound is explosive; for this reason, diazonium salts are prepared in ice-cold solutions and are used immediately.
They are synthesised in a diazotisation reaction- a cold solution of sodium nitrate is added to a solution of arylamine in concentrated acid (below 5^oC).
The acid firstly reacts with the sodium nitrate to form an unstable nitrous acid (nitric(iii) acid):
NaNO_2(aq) + HCl_(aq) HNO_2(aq) + NaCl_(aq)
The nitrous acid then reacts with the arylamine:

In a diazo coupling reaction, the doazonium salt reacts with another arene (the coupling agent).
The diazonium salt acts as an electrophile, reacting with the benzene ring of the coupling agent.
When the ice-cold solution of the diazonium salt is added to a solution containing the coupling agent, a coloured precipitate of an azo compound is formed; many of these compounds are dyes.
The coupling agent always reacts in the two or four position of the benzene ring (where one position is the functional group).
The colour of the compound formed depends on the coupling agent that is being reacted with diazonium salt.

When the diazonium salt is reacted with a phenol, a yellow/orange azo compound is formed:
With an alkaline solution of napthalein-2-ol, a red azo compound is formed:

A yellow dye is often formed when a diazonium salt is reacted with arylamines:
Many different azo compounds can be formed by coupling different diazonium salts with a range of coupling agents.
The azo compounds are stable and so their colours do not fade.