Structures of Pyrrole, Furan, and Thiophene

The classical structures of pyrrole, furan and thiophene (31) suggest that these compounds might show chemical reactions similar to those of amines, ethers and thioethers (32) respectively. On this basis, the initial attack of the electrophile would be expected to take place at the heteroatom and lead to products such as quaternary ammonium and oxonium salts, sulfoxides and sulfones. Products of this type from the heteroaromatic compounds under consideration are relatively rare. 

Five-Membered Unsaturated Heterocycles 1151 Structures of Pyrrole, Furan, and Thiophene 1152 Electrophilic Substitution Reactions of Pyrrole, Furan, and Thiophene 1153 Pyridine, a Six-Membered Heterocycle Electrophilic Substitution of Pyridine Nucleophilic Substitution of Pyridine Fused-Ring Heterocycles 1158 Nucleic Acids and Nucleotides 1160 Structure of Nucleic Acids 1163 Base Pairing in DNA The Watson-Crick Model Nucleic Acids and Heredity 1166 Replication of DNA 1167... 

Pyrroles, furans and thiophenes react preferentially with free radicals at the 2-position. Thus, reaction of pyrrole with benzyl radicals gives 2-benzylpyrrole. With triphenylmethyl radicals, pyrrole behaves like butadiene giving the adduct (163). /V-Methylpyrrole undergoes free radical benzoyloxyla-tion with dibenzoyl peroxide to give the 2-benzoyloxypyrrole (164) and 2,5-dibenzoyloxypyrrole (165). Furan, however, is converted in good yield to a mixture of cis and trans addition products analogous in structure to (163). 

Bonino s first results in this direction were published in four papers, three in Zeitschrifi fur Physikalische Chemie in 1933 and 1934 and one in Memorie della Reale Accademia delle scienze dell istituto di Bologna. [45] Through the study of the constitution and the aromatic character of the heterocyclic compounds, Bonino confronted the classical ideas of structural organic chemistry. In the case of heterocyclic compounds he and his collaborators emphasized that the classical structure formulae of organic chemistry could not account for the new Raman spectroscopic data. According to these, the existence of a double chemical bond for pyrrole, furane, and thiophene was improbable. 

The differing amounts of aromatic stabilization for benzene, pyrrole, furan, and thiophene demonstrate that aromatic stabilization occurs in varying degrees, depending on the structure of the compound. Some compounds have a large aromatic stabilization that dramatically affects their stabilities and chemical reactions. Others may have only a small stabilization and have stabilities and reactions that are more comparable to a normal alkene. 

Instead, these heterocycles and their derivatives most commonly undergo electrophilic substitution nitration, sulfonation, halogenation. Friedel-Crafts acylation, even the Reimer-Tiemann reaction and coupling with diazonium salts. Heats of combustion indicate resonance stabilization to the extent of 22-28 kcal/ mole somewhat less than the resonance energy of benzene (36 kcal/mde), but much greater than that of most conjugateci dienes (about Tlccal/mole). On the basis of these properties, pyrrole, furan, and thiophene must be considered aromatic. Clearly, formulas I, II, and III do not adequately represent the structures of these compounds. 

Pyrrole, furan, and thiophene are l-hetero-2,4-cydopentadienes. Each contains a butadiene unit bridged by an -hybridized heteroatom bearing lone electron pairs. These systems contain delocalized tt electrons in an aromatic six-electron framework. This section considers the structures and methods of preparation of these compounds. 

Chemical vapor deposition (CVD) of some organic compounds was examined to control the porosity and surface function of active carbon fiber (ACF). In this system, the deposition takes place only on the pore wall of the ACF, when the precursor organic compound and deposition temperature around 700 C were selected carefully. The surface of the ACF was modified by carbon derived from heterocyclic compounds (pyridine, pyrrole, furan and thiophene) through CVD. The moderately activated ACF modified by pyridine, pyrrole and thiophene showed molecular sieving activity, that modified by furan did not. Only fiiran was decomposed at this temperature. Thermal stability is a key factor to get molecular sieving performance after CVD. Pyridine produced amorphous carbon within the pore, which appears to maintain the pyridine ring structure, creates basic sites over the surface of the ACF. Thus catalytic oxidation of SOx over ACF of high surface area was accelerated. 

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