The nitrogen lone pair in pyrrole forms part of the aromatic 6-electron system. In this structure, the charge distribution in the resonance canonical structures (responsibles of the mesomeric effect) makes that electron density drifts away from the nitrogen. On the other hand, the inductive effect of the nitrogen is towards the hetero atom and away from carbon, meaning that electronic distribution in pyrrole is a balance of two opposing effects, of which the mesomeric effect is often the most significant. As a consequence, de dipolar moment of the pyrrol is solvent dependent and away from the N towards the carbon. For this reason, five-membered heterocycles of the pyrrole type are referred as electron-rich or, better, pi-electron-rich.
For thiophene and furane, however, the heteroatom's higher electronegativity means that positive charges on the hetero atom in canonical resonance make a smaller influence. The decreased mesomeric electron drift away is insufficient, in these cases, to overcome inductive polarisation and the dipolar moment is practically not dependent on the solvent and and away from the N and away from carbon.
Due to their higher pi-electron richness, heterocycles such as pyrrole, thiophene and furan undergo electrophilic substitution much easier than their 6-membered counterparts. And although either carbon on the ring is possible, the preference is the attack to the adjacent hetero atom (alfa position). As this preference is facilitated by the electron-release from the hetero atom, pyrroles are more reactive than furans which are in turn more reactive than thiophenes. Indoles are only slightly less reactive than pyrroles, electrophilic substitution taking place preferably in the heterocyclic ring.
The reactivity of an indole is very comparable to that of a phenol: depending on pH, indoles can undergo coupling reaction at the heteroatom with certain easiness.
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