yhenesrpreparedrom a 6热n OcanaettCeu X-ME.A5 4Reateenonenes 6ecoophf2anaadec8hiniondergo 24eveoemea8enesene8m0at8nCsnhe1hete 一孕-9到 akddit orm and ald be 82yo2 内eak b Reehdly Ac 3 3 As a consequence of electron delocalization, pyrrole, furan and thiophene exhibit unusual stability, deshielded protons in their 1H NMR spectra due to ring currents and the ability to undergo electrophilic aromatic substitution. The electron delocalization in pyrrole can be described by resonance structures: Based on the charge distribution, the heteroatom should be electron poor and the ring carbons electron-rich. Pyrroles, furans and thiophenes are prepared from γ-dicarbonyl compounds. A general approach for the synthesis of heterocyclopentadienes is the Paal-Knorr synthesis (pyrroles) and its variations (for the other heterocycles). An enolizable γ-dicarbonyl compound is treated with an amine derivative (for pyrroles), P2O5 (for furans) or P2S5 (for thiophenes). Reactions of the Aromatic Heterocyclopentadienes 25-4 Pyrroles, furans and thiophenes undergo electrophilic aromatic substitution. There are two possible sites of attack, C2 and C3, in the 1-hetero- 2,4-cyclopentadienes undergoing electrophilic substitution. Attack at C2 leads to an additional resonance form and should be the preferred site of attack. Attack at C2 is generally observed, however, because C3 is also activated to electrophilic attack, mixtures of products may occur depending upon conditions, substrates and electrophiles. The relative nucleophilic reactivity series is: benzene << thiophene < furan < pyrrole Pyrrole is very non-basic compared to ordinary amines. Protonation occurs at C2 rather than N, and then only with very strong acids. Pyrroles are in fact relatively acidic. After losing a proton, the nitrogen atom rehybridizes from sp3 to sp2 and the negative charge becomes delocalized, as in the cyclopentadienyl anion