Chemguide: Support for CIE A level Chemistry


Learning outcome 20.1(e)

This statement 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 statement in your copy of the syllabus.


This statement will only be examined in the final exam of a two year course.


You will find what you need to know on the page reactions of diazonium salts.

There is far more on this page than you actually need:

  • Ignore the first section on substitution reactions.

  • In the section on coupling reactions, concentrate on the first part about the reaction with phenol, because that is what the syllabus asks for. Notice that the coupling takes place in the 4- position, opposite the oxygen in the phenoxide ion. That will always be the case unless the 4- position is already occupied.

  • Now read through the other coupling reactions involving naphthalen-2-ol (2-naphthol) and phenylamine, but ignore the more complicated case of methyl orange. Notice that the 4- position isn't available in the 2-naphthol case, and so the coupling happens on the position next-door to the oxygen.

    The reason that it chooses this particular next-door position is beyond anything that CIE will ever ask.

    You don't specifically have to learn these other examples, but it is important that you can see how these cases relate to the phenol one.

    It is quite likely that CIE will come up with other molecules that a diazonium ion can couple with, and ask you to draw the structure of the product. It is essential that you can get the structure of the -N=N- bridge right, and preferable if you could get the link in the right 4- position (if that is available).


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© Jim Clark 2011 (last modified June 2014)