Thiophenes aromaticity

Whereas the initial hydrogenation both breaks a K bond, as well as destroys any aromatic stabilization, the second hydrogenation only breaks the k bond. Is this difference large as in benzene (see discussion at right), or is it much smaller or neglible Is thiophene aromatic ... 

Thiophene, a sulfur-containing heterocycle, undergoes typical aromatic substitution reactions rather than addition reactions. Why is thiophene aromatic ... 

Thiophene, aromaticity of, 530 Thiourea, reaction with alkyl halides, 667... 

Loss of C2H2 from thiophene Aromatic compounds Pyrrole... 

The term charge tranter refers to a succession of interactions between two molecules, ranging from very weak donor-acceptor dipolar interactions to interactions that result in the formation of an ion pair, depending on the extent of electron delocalization. Charge transfer (CT) complexes are formed between electron-rich donor molecules and electron-deficient acceptors. Typically, donor molecules are p-electron-rich heterocycles (e.g., furan, pyrrole, thiophene), aromatics with electron-donating substiments, or compounds... 

On the other hand, benzannulation in the 3,4 positions of PT, would produce the nearly degenerate structures 2a and 2b (Scheme I) because the gain in aromaticity of the benzene ring"Tn 2b is expected to outweigh the loss of thiophene aromaticity in 2a. The result would be amelioration of bond alternation ("Peierls distortion" of the backbone) with a concomitant decrease in bandgap. 

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... 

A good and comprehensive review of catalytic electrophilic acylation was published by Pearson and Buehler.i Only the catalysts most widely used were considered, with special attention to iron trichloride, zinc chloride, iodine, and elemental iron. The substrates that can be acylated using small amounts of catalysts include alkylarenes, aryl ethers, biphenyls, naphthalenes, acenaphthenes, fluorene, furans, and thiophenes. Aromatic acyl chlorides lead to better yields than aliphatic ones, reaching a maximum of 96% and a minimum of 34%. In general, fhe reactions are performed af relatively high temperatures (from 50°C to 200°C) af which hydrogen chloride evolution is fairly rapid. 

All electrochemically stable metal complexes that are substituted by pyrrole, thiophene, aromatic amino, aromatic phenol and some other groups such as vinyl, are suitable for anodic electropolymerizations from solution (organic solvent or water) in the presence of a conducting salt (counter ions in the oxidized films). 

Polyisothianaphthene (PITN), Fig. 23, is also a polythiophene derivative where all the thiophene rings are fused to a benzene ring at the 3,4-positions of thiophene. It possesses a band gap of 1 eV, which is smaller than that of polythiophene. The PITN main chain favors the quinoid form to preserve aromaticity causing a change in the thiophene aromaticity [140]. 

In the Asteraceae, acetylenes are widely distributed and structurally very diverse, including aliphatic acetylenes, acetylenic thiophenes, aromatics, isocoumarins and spiroacetal enol ethers (Table 5.1 and Figures 5.3,5.4 and 5.5). 

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