Chemguide: Support for CIE A level Chemistry

34.2: Phenylamine and azo compounds

Learning outcome 34.2.4

This series of statements is about the reactions of diazonium ions in compounds such as benzenediazonium chloride with molecules like phenol to produce azo dyes.

Before you go on, you should find and read the statements in your copy of the syllabus.

Learning outcome 34.2.4(a)

Start by reading the beginning of the "Coupling reactions" section on the page reactions of diazonium salts down to and including the reaction with naphthalen-2-ol.

You do not need to know about substitution reactions for this statement, but you might need the reaction with naphthalen-2-ol as a part of statement 32.2.7.

Learning outcome 34.2.4(b)

The azo group is the -N=N- link.

Learning outcomes 34.2.4(c) and 34.2.4(d)

You have to know that azo compounds are often used as dyes and can be formed in the same way as the coloured product in the phenol reaction you have just met.

Now read through the other coupling reactions involving naphthalen-2-ol (2-naphthol) and phenylamine on the page above, but ignore the more complicated case of methyl orange (unless you are interested). You will not need to learn either of these - they are simply an illustration of the syllabus statement.

Questions that CIE have asked on this in the past have been straightforward. For example, they might ask you about an unknown compound which they treated with ice-cold nitrous acid followed by an alkaline solution of phenol to produce a coloured compound.

They would expect you to know that the unknown compound must have an NH₂ group attached to a benzene ring. That's not hard!

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