Studies of Azo Colors. XI. The Diazo-coupling Reaction of Pyrroles

Kinetic studies have been made of the coupling reaction of several diazonium salts with pyrrole, N-methylpyrrole, and N-ethoxycarbonylpyrrole. In the case of pyrrole, it was found, from the dependence of the apparent rate constant on the pH values, that the undissociated pyrrole reacted with diazonium salts in the pH range from 4.7 to 8.2, whereas in the pH region higher than 10.0 the reactive species was the conjugate base of pyrrole. The reactivity of the pyrrole anion was higher by a factor of 8 powers of ten than that of the undissociated pyrrole. The effects of the substituents of diazonium salts on the reaction rates were also investigated. The ρ-values (4.3–4.6) obtained in the Hammett plots were practically the same as those of diazo-coupling reactions which involve ordinary aromatic amines as coupling components.