Diazo CouplingThus the 2,4-dinitrophenyldiazonium cation will couple with
Ph-OMe and the 2,4,6-compound with the hydrocarbon mesitylene.
Diazonium cations exist in acid and slightly alkaline solution (in more
strongly alkaline solution they are converted into diazohydroxides,
PhN=N—OH and further into diazotate anions, PhN=N- O^0 )
and coupling reactions are therefore carried out under these conditions,
the optimum pH depending on the species being attacked. With
phenols this is at a slightly alkaline pH as phenoxide ion is very much
more rapidly attacked than phenol itself because of the considerably
greater electron-density available to the electrophile:
Coupling could take place on either oxygen or carbon and though
relative electron-density might be expected to favour the former, the
strength of the bond formed is also of significance and as with
electrophilic attack on phenols in general it is a C-substituted product
that normally results:
The proton is removed by one or other of the basic species present in
solution.E^113enhance its electrophilic character, however, by increasing the
positive charge on the diazo group: