2.3 - Structure of Five-Membered Rings with One Heteroatom

Abstract

This chapter focuses on five-membered heterocyclic compounds, and discusses their possible structures, nomenclature, properties, and tautomerism. The simplest nitrogen heterocyclic analogue of the aromatic cyclopentadiene anion is pyrrole, while the corresponding oxygen and sulfur analogues are furan and thiophene. The benzo-fused pyrrole heterocycles are analogues of naphthalene with carbazole having an electronic relationship to phenanthrene. There are four types of fully conjugated five-membered rings with exocyclic conjugation. The heteroatoms (N, O, and S) in pyrrole, furan, and thiophene contribute two π-electrons to the aromatic sextet and push electron density toward the ring carbons. As a result, the carbon atoms in the ring acquire partial negative charge and pyrrole, furan, and thiophene are described as electron-rich or π-excessive. Hückel molecular orbital (HMO) calculations give a measure of the distribution of the π-electrons; however the method is less reliable for five-membered rings than for six-membered rings. Optimized geometries have been calculated for many pyrrole, furan, and thiophene derivatives using a variety of MO methods. Structures of five-membered heterocyclic compounds can be derived from methods such as X-ray diffraction, spectroscopic methods (including UV, NMR, and microwave spectroscopies), photoelectron spectrometry, and mass spectrometry. Pyrrole, furan, and thiophene have limited solubility in water; their aqueous solubilities are 6, 3, and 0.1%, respectively. All of the parent fully conjugated five-membered heterocycles possess some degree of aromaticity based on their chemical behavior, such as a tendency to undergo substitution reactions with electrophilic reagents. Prototropic tautomerism is the only significant type of tautomerism of five-membered heterocycles.