Pyrryl anion

Removal of the proton from an azole iV-hydrogen generates an N-anion, for example the pyrryl anion. Such species are stiU aromatic, but now have a lone pair of electrons at the nitrogen, in an sp hybrid orbital, in the plane of the ring and not part of the aromatic sextet. 

The high reactivity of pyrroles is illustrated by their ready reaction with benzenediazonium salts. Pyrrole itself gives a mono-azo derivative by reacting as a neutral species below pH 8, but by way of the pyrryl anion (section 13.4), and 10 times faster, in solutions above pH 10. In more strongly alkaline conditions 2,5-bisdiazo derivatives are formed. 

Pyrrole A-hydrogen is much more acidic (pAa 17.5) than that of a comparable saturated amine, say pyrrolidine (pAa 44), or aniline (pAa 30.7), and of the same order as that of 2,4-dinitroaniline. Any very strong base will effect complete conversion of an A-unsubstituted pyrrole into the corresponding pyrryl anion. 

---