ABSTRACT
This chapter discusses the most important generalisations which can be made, and the general lessons which can be learned about the reactivity, and relative reactivities, one with the other, of the prototypical five-membered, aromatic heterocycles: pyrroles, thiophenes and furans. The occurrence of such processes in the case of furan is generally considered to be associated with its lower aromatic resonance stabilisation energy there is less to regain by loss of a proton and the consequent return to an aromatic furan. Pyrroles which have an N-hydrogen are deprotonated at the nitrogen, and the pyrryl anion thus generated is nucleophilic at the hetero atom, providing a means for the introduction of electrophilic groups on nitrogen. An aspect of the chemistry of furans is the occurrence of a number of 2,5-additions initiated by electrophilic attack: a Wheland intermediate is formed normally but then adds a nucleophile, when a sufficiently reactive one is present, instead of then losing a proton.
