Structure and Physical Properties of Thiophenes

Since the time of the quantum-mechanical calculations by Longuet-Higgins, many attempts have been made to calculate tt-electron densities, resonance energies, dipole moments, and optical transitions both by the LCAO-MO and the valence bond method.However, no agreement has been reached on the importance of pd-hybridization of the sulfur atom. This is considered by some workers an essential 

Quantum-mechanical calculations on substituted thiophenes are still more difficult and localization energies and electronic charges have been calculated only for 2- and 3-nitrothiophene.  

Gronowitz et al. have discussed the effects of substituents on chemical reactivity and on ultraviolet (XJV), infrared (IR), and nuclear magnetic resonance (NMR) spectra in terms of simple resonance theory,They assume resonance structures (1-5) to contribute to a —I—M (Ingold s terminology) 2-substituted thiophene, resonance forms (6-10) to the structure of a drI-fM 2-substituted thiophene, forms (11-16) to a —I—M 3-substituted thiophene, and forms (17-22) to a I -M 3-substituted thiophene. 

The foregoing resonance structures describe the influence of the substituents on the 7r-electron distribution in the thiophene ring. Besides this effect the inductive effect of the substituents on the a-electi on system must be considered when discussing physical and chemical properties of thiophenes. 

Physical and chemical evidence supporting the theory mentioned in the foregoing will be given in the appropriate sections. Some predic-. tions may be mentioned here. From resonance structures (2, 3, 7, 8) it is inferred that the substituent effect in 2-substituted thiophenes should be parallel to that in the corresponding benzenes, the 3- and 5-positions may be considered as ortho and para positions and the 4-position as a meta position. It is, however, obvious that the effect of a —M- and a d-M-substituent are not simply reversed, as reso- 

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III. Molecular Structure and Physical Properties of Benzo[6]thiophenes... 

Sulfur oxidation of the thiophene ring leading to thiophene-1-oxides or thiophene-1,1-oxides is useful for modifying the chemistry of the thiophene ring and this subject has been reviewed <02CHC632>. The structure and physical properties of quinquethiophene-1,1-dioxides have been studied in detail <02T10151>. The mechanism for the oxidation of thiophene to a mixture of thiophene-l-oxide (trapped as a Diels-Alder adduct) and thiophen-2-one was studied using... 

Ch. 2 Biological and Pharmacological Activities of Thiophene and Its Derivatives Ch. 3 Syntheses and Physical Properties of Thiophene and Its Derivatives Ch. 4 Molecular Structure and Spectroscopy of Thiophene and Its Derivatives Ch. 5 Factors Affecting Substitution Reactions in the Thiophene Nucleus... 

In the discussion of structural and spectroscopic aspects of thiophenes to follow, the reader does well to bear in mind that in fact aromaticity is a hallmark of thiophenes that dominates their properties and governs their reactions. Moreover, thiophene stands as uniquely aromatic among the five-membered heterocycles. In concrete terms this means that not only does thiophene undergo the reactions and have the physical and spectral properties associated with the concept of aromaticity, but that this aromatic character is steadfastly maintained through all manners of transformations including fusion with other aromatic rings. This attachment of substituents or fusion with other rings spawns thousands of aromatic derivatives. 

Thiophenes and their Benzo Derivatives Structure Table 1 Physical properties of thiophene <94MI 209-01). 

In this chapter we will first discuss the synthesis, chemistry and physical properties of silicon-containing thiophene monomers, oligomers and polymers. We will first summarize the synthesis of silylated thiophene derivatives. The use of selective silylation reactions allows the design of thiophene monomers and oligomers with various structural features. Some of the properties of these new conjugated structures will be discussed. 

Although physical characterizations have played an important role in the progress of our knowledge of the structure and electronic properties of PTs, the major driving force of their development lies in the considerable research efforts that organic chemists have invested in the control of the structure and properties of thiophene-based polymeric or molecular conjugated systems. 

It has been mentioned already that polypyrrole (25) and polythiophene (26) play an important role as electrical conductors and polymeric anodes in battery cells [2,47,226]. Since the charging and discharging of the conjugated polymer is accompanied by the incorporation and removal of counterions it is clear that the material can also act as a carrier of chemically different anions which influence the physical, chemical and physiological properties of the material [292]. With regard to the full structural elucidation of the polymers it must be added, however, that the electropolymerization process of pyrrole and thiophene does not provide a clean coupling of the heterocycles in the 2,5-positions. Instead, the 3- and 4-position can also be involved giving rise to further fusion processes under formation of complex polycyclic structures [47]. 

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]. 

The thiophene hydrodesulfurization and cyclohexene hydrogenation activity of the catalysts have been measured in a high pressure reactor, the operative conditions having been selected according to the recent experience about the CoMoe and NiMoe based systems because of the common structural and physical-chemical properties [3, 4]. Table 3 shows chemical data and conversion obtained for selected catalysts based on RhMoe. In addition, the data for CoMoe Anderson, CoMo commercial and Rh commercial 7-AI2O3 supported catalysts are included for comparative purposes. 

Physical Properties of Benzo[hJ thiophens. The aromatic electronic delocalization in benzo[h] thiophen has been calculated.The m.c.d. spectra of benzo[h]thiophen and dibenzothiophen have been analysedChemical thermodynamic properties of benzo[h]thiophen and other condensed systems have been determined. The crystal structures of 5-nitroso-4-(phenylethyl)benzo[h] thiophen, 1,2,3,5-tetramethylbenzo[h] thiophenium tetrafluoroborate, " and a spiro-benzo[Z)]thieno[2,l-h]pyran have been determined. 

This approach is based on the use of the unique physical properties of surfactant molecules in aqueous solutions, which n es it possible to electropolymerize thiophene derivatives and to study the influence of micelles on the electropolymerization process and on the structures and properties of die resulting polymer films. 

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