Heterocyclic compounds, aromatic thiophenes

Aromatic sextets can also be present in five- and seven-membered rings. If a five-membered ring has two double bonds and the fifth atom possesses an unshared pair of electrons, the ring has five p orbitals that can overlap to create five new orbitals— three bonding and two antibonding (Fig. 2.6). There are six electrons for these orbitals the four p orbitals of the double bonds each contribute one and the filled orbital contributes the other two. The six electrons occupy the bonding orbitals and constitute an aromatic sextet. The heterocyclic compounds pyrrole, thiophene, and... 

Cyclic compounds that contain at least one atom other than carbon within their ring are called heterocyclic compounds, and those that possess aromatic stability are called het erocyclic aromatic compounds Some representative heterocyclic aromatic compounds are pyridine pyrrole furan and thiophene The structures and the lUPAC numbering system used m naming their derivatives are shown In their stability and chemical behav lor all these compounds resemble benzene more than they resemble alkenes... 

The species which are unknown and have not been identified as one of the major chemical lump such as alkanes, phenols and aromatics are lumped together as unidentified. However, the species in this lump include saturated and unsaturated cycloalkanes with or without side chains, which resembles the naphthenes, a petroleum refinery product group. A number of well known species in coal liquid are not mentioned in this lumping scheme. Such as heterocyclic compounds with sulfur, nitrogen or oxygen as the heteroatom, and other heteroatora containing species. Some of these compounds appear with aromatics (e.g. thiophenes, quinolines) and with phenols (e.g. aromatic amines), and most of them are lumped with the unidentified species lump. 

Leonid Belen kii was born in Moscow, and he graduated from M. V. Lomonosov Moscow State University in 1953 with Professor A. P. Terentiev as supervisor in organic chemistry. Since 1955, he has worked as junior, senior (since 1966), and leading scientist (since 1988) at N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, where he obtained his Ph.D. degree (1963) under the direction of Professor Ya. L. Gol dfarb and his Degree of Dr. Chem. Sci. (1974) and rank of Professor in Chemistry (1991). His scientific interests include all aspects of chemistry of heterocyclic and aromatic compounds, particularly electrophilic substitution in benzene, thiophene, furan, and azole series as well as organosulfur chemistry. 

Refining engineers and chemists are most interested in the ease of desulfurizing petroleum using thermal and thermocatalytic treatments. The sulfur is removed primarily as hydrogen sulfide. Thermal and thermocatalytic studies have established that non-thiophenic sulfur (aliphatic as in thiols, acyclic and cyclic sulfides) evolve H2S much more readily than thiophenic sulfur (aromatic heterocyclic compounds). Thus, the relative abundances of nonthiophenic (aliphatic) and thiophenic (aromatic) sulfur is a critical characteristic for all fuels with respect to ease of desulfurization. Analytical methods were developed in the 1960s for classifying the total sulfur in crude... 

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. 

This behavior may be due to different reasons. First, the different aromaticity of the compounds could play an important role to define the reactivity of the compounds. Furan is the lowest aromatic pentaatomic heterocyclic compound known, while the other compounds show higher aromaticity. However, this type of explanation cannot justify why thiophene does not react while simple dimethylthienyl derivatives react and why some dimethylthienyl derivatives react while some others do not show any reactivity. 

The aromaticity of a heterocycle depends on how effectively the lone-pair of the heteroatom contributes to the aromatic sextet. The aromaticity of five-membered heterocyclic compounds may be estimated from their reactivity in the Diels-Alder reaction.94 Spectrophotometry shows that furan, thiophene, and selenophene resemble benzene in that with maleic anhydride 1 1 complexes are formed which are stable up to 150°C in the case of thiophene, decompose at 150°C with selenophene (whereby selenium is formed together with a diene which gives a further adduct with another molecule of maleic anhydride), and produce the usual adduct at 20°C with furan. Thus, only furan is a normal diene as regards the Diels-Alder reaction. 

This may imply that the intermolecular coupling of various aryl halides with other heteroaromatic compounds may proceed. Indeed, it is now known that not only the special heteroaromatic halides but also usual aryl halides can react with a variety of five-membered aromatic heterocycles, including furans, thiophenes, and azole compounds such as M-substituted imidazoles, oxazoles, and thiazoles [133-137]. The arylation of azoles can be carried out using iodobenzoate immobilized on an insoluble polymer support [138]. Related intermolecular reactions of indole [139] and imidazole [140] derivatives have also been reported. 

The anodic methoxylation of aromatic compounds such as naphthalene [41], anthracene [42], alkylbenzenes [31,43], phenols [44-46], anisoles [33,47-54] and other alkoxyben-zenes [53], methoxynaphthalenes [33], methoxyanthracenes [50,54], inden-l-ones [55], / a/r/-substituted anilides [56] and heterocyclic compounds, such as furans [57], thiophenes [58], and pyrroles [59], has received considerable attention. 

Mossner, S.G., Lopez de Alda, M.J., Sander, L.C., Lee, M.L. and Wise, S.A. (1999). Gas Chromatographic Retention Behavior of Polycyclic Aromatic Sulfur Heterocyclic Compounds, (diben-zothiophene, naphtho[b]thiophenes, benzo[b]naphthothiophenes and alkyl-substituted derivatives) on Stationary Phases of Different Selectivity. J.Chromat., 841,207-228. 

Furan, thiophene, pyrrole, and pyridine are all examples of heterocyclic aromatic compounds (heteroaromatic compounds). The heteroatoms in some of these compounds (furan, thiophene, pyrrole) contribute one lone pair to the aromatic system, whereas in others (pyridine) they contribute none. You can determine how many lone pairs a heteroatom contributes to the aromatic system by examining the effect of lone-pair donation on the hybridization of the heteroatom. For example, if the N atom of pyridine used its lone pair to participate in resonance, it would have to be sp-hybridized (one p orbital required for the N=C n bond, one for the lone pair used in resonance), but sp hybridization requires 180° bond angles, which are not possible in this compound. Therefore the N atom must be sp2-hybridized, and the N lone pair must be in a hybrid orbital that is orthogonal to the cyclic array of p orbitals. In pyrrole, by contrast, if the N atom uses its lone pair in resonance, the N atom must be sp2-hybridized, which is reasonable. Therefore, there is a cyclic array of p orbitals in pyrrole occupied by six electrons (two from each of the C=C it bonds and two from the N lone pair), and pyrrole is aromatic. 

After a brief survey of the history of valence-bond isomers of aromatic compounds, new syntheses and the reactions of these isomers reported in the last decade are reviewed. In the second chapter, the valence-bond isomers of homoaromatic compounds, especially benzene derivatives, are described and in the third chapter those of heterocyclic compounds. Photoreactions of perfluoroalkylated aromatic compounds afford valence-bond isomers in high yields. These isomers are very stable and useful for the synthesis of highly strained compounds. Therefore, the emphasis is put on the chemistry of trifluoromethylated benzvalenes, Dewar thiophenes, and Dewar pyrroles. 

The ASE values correlate with magnetic susceptibility for the five-membered heteroaromatic compounds. Magnetic and polarizability criteria put the order of aromaticity as thiophene > pyrrole > furan. The other criteria of aromaticity discussed in Section 8.2 are also applicable to heterocyclic compounds. HOMO-LUMO gaps and Fukui functions (see Topic 1.5) have been calculated for compounds such as indole, benzofuran, and benzothiophene and are in accord with the known reactivity patterns of these heterocycles. 

The reaction is applicable to benzene, benzene derivatives,174-176,183,184 polycyclic aromatic hydrocarbons,185 and also heterocyclic compounds of aromatic character such as thiophen, furan, and pyridine Volhard, for instance, in 1892 mercurated thiophen directly with mercuric chloride.186... 

The Friedel-Crafts synthesis with acyl chlorides and anhydrides can be applied to all aromatic hydrocarbons and to many heterocyclic compounds, in particular to thiophene, furan, and pyrrol derivatives. Pyridine does not react,531,532 and hydrocarbons carrying electron-attracting substituents react with difficulty or not at all. 

An annulene is a monocyclic hydrocarbon with alternating single and double bonds. A heterocyclic compound is a cyclic compound in which one or more of the ring atoms is a heteroatom—an atom other than carbon. Pyridine, pyrrole, furan, and thiophene are aromatic heterocyclic compounds. 

The role of heteroatoms in ground- and excited-state electronic distribution in saturated and aromatic heterocyclic compounds is easily demonstrated by a comparison of a number of heteroaromatic systems with their perhydro counterparts. In Jt-excessive heteroaromatic systems, because of their resonance structures, their dipole moments are less in the direction of the heteroatom than in the corresponding saturated heterocycles furan (1, 0.71 D) vs. tetrahydrofliran (2, 1.68 D), thiophene (3, 0.52 D) vs. tetrahydrothiophene (4, 1.87 D), and selenophene (5, 0.40 D) vs. tetrahydroselenophene (6, 1.97 D). In the case of pyrrole (7, 1.80 D), the dipole moment is reversed and is actually higher than that of pyrrolidine (8, 1.57 D) due to the acidic nature of the pyrrole ring (the N-H bond) In contrast, the dipole moment of n-deficient pyridine (9, 2.22 D) is higher than that of piperidine (10, 1.17 D). In all these compounds, with the exception of pyrrole (7), the direction of the dipole moment is from the ring towards the heteroatom [32-34]. 

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