5petegaoa2oe8ntadeasaionundergoring ietrreesaopaeatieaeiegeuupoaRaneynlcke 的 5ben2xpoacgigtreBcse5ctuicentdene under theserecihere ndole is a benzopyrrole. 25-5 Structure and Preparation of Pyridine:an Azabenzene dine is aromatic. ne is 点8wa e4 1-Hetero-2,4-cyclopentadienes can undergo ring opening and cycloaddition reactions. Hydrolysis of furans to γ-dicarbonyl compounds under mild conditions can be viewed as the reverse of the Paal-Knorr-type synthesis of furans. Pyrrole polymerizes under these reaction conditions, whereas thiophene is stable. Sulfur-free acyclic compounds are formed upon Raney nickel desulfurization of thiophene derivatives. Furan, but not pyrrole or thiophene, possesses sufficient diene character to undergo Diels-alder cycloadditions. Indole is a benzopyrrole. Indole is the most important benzannulated derivative of the 1- herero-2,4-cyclopentadienes. It is found in many natural products including the amino acid tryptophan. Indole is related to pyrrole in the same way that naphthalene is related to benzene. It possesses many possible resonance forms, however those disturbing the cyclic six-π-electron of the fused benzene ring are less important. Structure and Preparation of Pyridine: an Azabenzene 25-5 Pyridine is aromatic. Of the two lone-pair electrons on the nitrogen atom, only one is in the p orbital perpendicular to the ring. The other is in the otherwise empty sp2 orbital pointing away from the ring. The nitrogen atom in pyridine is sp2 hybridized like the nitrogen atom in an imine. As a result, nitrogen does not donate excess electron density into the ring, but withdraws electron density, both inductively and by resonance. The 1H NMR spectrum indicates the presence of a ring current. The chemical shifts for the protons at C2 and C4 are more deshielded as is expected from the resonance picture. Pyridine is a weak base. The nitrogen lone pair is not tied up by conjugation as it is in pyrrole. The pKa of pyridinium is lower than that of an alkanammonium ion (~10) because the nitrogen atom is sp2 hybridized, not sp3 hybridized