Structures of Thiophene and Furan

Data on the physical and chemical properties of PCDTs and PCTAs are scarce. Due to their structural similarity to PCDFs and PCDDs they are also supposed to possess some likeness in their physical and chemical properties. Sulfur and oxygen are both Group VI elements with two outer shell electrons available for covalent bonding. Structures of thiophene and furan with benzene carbon-sulfur (Cb-S) and carbon-oxygen bond (Cb-0), in PCDTs and PCDFs respectively, suggest similar chemical behavior. The bond dissociation energies (AH) show that less energy is required to break the Cb-S bond than the Cb-0 bond [17,36,37]. 

Figure 57 (a) Molecular structures of thiophene and furane-appended fuUerene derivatives I-III (b) schematic illustration of the... 

Oxygen can be present in naphthenic acids, phenols and furan (analogue structures to thiophene and pyrrole) and higher derivatives. Oxygenated compounds give rise to corrosion and product deterioration. Hydrodeoxygenation is particularly important in the upgrading of biomass. 

The data shown in Table 2 illustrate the general paucity of comparative toxicity data within an isosteric series of chemicals. In this Table a variety of toxic end-points observed for benzene and naphthalene have been compared with those of their simple heterocyclic analogues, and it is clear that it is almost impossible to derive chemical structure-biological activity relationships from the published literature for even such a simple series of compounds. Even basic estimates of mammalian toxicity such as LD50 values cannot be accurately compared due either to the absence of relevant data or the noncomparability of those available. Thus in a field where there are little comparative data on the relative toxicity to mammals of pyrrole, thiophene and furan for example, it is difficult to relate chemical structure to biological activity in historical heterocyclic poisons such as strychnine (3) and hemlock [active agent coniine (4)]. 

The structures below summarize hydrogen activities of selenophene, thiophene, and furan in protophilic isotope exchange the rate constant of deuterium exchange at the a position of furan is arbitrarily given the value of unity. 

While dications 21 and 22 only underwent a one-stage two-electron reduction without formation of the radical cations, the reduction of the thiophene and furan derivatives proceeded in two steps, in the first of which the radical cations 23+ and 24+ were formed. The stability of these radicals was proved by the fact that the EPR spectrum of 23+, consisting of a strong line (g = 2.0038) with rather complicated hyperfine structure, persisted with undiminished intensity for weeks at room temperature. 

Thiophene is aromatic. Its electronic structure follows from Fig. 5.2 (see p 53). Thiophene is a n-excessive heterocycle, i.e. the electron densitiy on each ring atom is greater than one. The value of the empirical resonance energy of thiophene is approximately 120 kJ moT the Dewar resonance energy is quoted as 27.2 kJ mol The aromaticity of thiophene is thus less than that of benzene but greater than that of furan. There are two possible explanations to account for the difference between thiophene and furan ... 

Thiophene is an aromatic compound. Its structure can be assumed to be derived from benzene by replacement of two annular CH groups with sulfur. The sulfur atom in this five-membered ring acts as an electron donating heteroatom by contributing two electrons to the aromatic sextet and thiophene is thus considered to be an electron-rich heterocycle. The chemistry of thiophene and other five-membered heterocycles, for example pyrrole and furan, are consequently dominated by this effect. 

The parent Dewar thiophene (3) has been generated and trapped <85JA723>. When a solution of thiophene in furan (mole ratio, 1 10) was irradiated at 229 nm at 25°C, the two 1 1 adducts (4) and (5) were formed in a combined yield of 1.2% of thiophene consumed. The structure and stereochemistry of these two adducts have been elucidated by detailed NMR analysis. In order to prove that it was indeed a photolysis product that was trapped by furan and not a short-lived excited state of thiophene, the photolysis of thiophene alone was carried out at —170 °C in a glassy... 

Polyynes (polyacetylenes). Compounds with very diverse structures containing several C/C triple bonds are produced mainly by fungi (basidiomycete cultures) and plants of the families Asteraceae, Apiaceae, and Araliaceae. In addition to conjugated triple bonds the R often also contain C/C double bonds, allene units, thiophene and furan rings. On account of the close biosynthetic relationships between these compounds, the term R is used as a collective name even when only one C/C triple bond is present in the molecule. As result of the work of Bohimann, E. R. H. Jones, Sorensen, and others more than one thousand natural R are now known. The antibiotically active mycotnycin (C H, g02, Mr 198.22, mp. 75 °C) from basidiomycete cultures, dehydromatricaria ester (CiiHgO, Mr 172.18, mp. 105-106°C) from Asteraceae, and the thiarubrins may be mentioned as typical examples (see also ter-thienyls). 

Thiophene (see Fig. 2), a five-membered aromatic ring containing sulfur, is comparable in structure to pyrrole and furan, which contain nitrogen and oxygen, respectively, but it possesses some unique qualities. Sulfur is an electron-donating heteroatom and it contributes two electrons to the 6jt electron-system of the ring and additionally has a lone pair of electrons in an sp -hybridized orbital, in the plane of the ring. Thiophene is thus an electron-rich aromatic heterocycle. 

Similar to furan and benzofuran monomers,thiophene and 1-benzothiophene " are also known to copolymerize in a 1 1 fashion with MA. Thiophene will not undergo free-radical homopolymerization or copolymerization with monomers such as styrene, methyl methacrylate, or acrylonitrile. However, when an equimolar mixture of thiophene and MA are combined with chloroform containing AIBN and heated at 60°C, a white copolymer starts to precipitate after only a few minutes reaction time. Varying the molar feed ratios showed that both yields and reduced viscosities were maximum for equimolar feeds. Copolymerizations in solution from 1 bar to 3 kbars show that rates and yields are increased with pressure.Through composition studies and comparing the NMR spectrum of the copolymer with the NMR spectra of 2,3- and 2,5-dihydrothiophene, structure 88 was assigned to the copolymer. 

One of the unique features of the alkene/CO copolymer is the existence of multiple carbonyl groups in the main chain. Thus, versatile chemical transformations of the carbonyl groups were examined to provide new polymers (Scheme 16). The 1,4-diketone structure ethylene/CO copolymers can be transformed into pyrroles, thiophenes, and furans upon treatment with primary amines,phosphorus pentasulfide, and phosphorus pentoxide, respectively. ... 

The major internal comparisons to be made within this chapter are between (13) pyrrole (1), furan (2), thiophene (3), selenophene (4) and tellurophene (5) b) pyrrole (1) and indole (6) (c) indole (6), benzo[6 Jfuran (7) and benzo[6]thiophene (8) d) indole (6), isoindole (9) and indolizine (10) and (e) benzo[6] and benzo[c] fused systems. The names of relevant heterocyclic radicals are given with the structures of the parent heterocycle. 

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. 

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