Degree of aromaticity

A comparison of the reactivity of the heterocycles, selenazoie, thiazole. and pyridine, was made by Ochiai (41), who used theoretical considerations to show that the degree of aromaticity was ... 

Typically, the oil is subjected to several successive oleum contacts ("treats") at ca 60—65°C. This charge ratio of feedstock oleum is generally empirically determined, but depends in part on the degree of aromatic removal required for the product. In the case of white oil manufacture, 100 parts of... 

The Jordi PDVB family of columns has very nonpolar packings with a high degree of aromatic character. They are made from the purest DVB available and currently serve as the base material for all other Jordi packings (Fig. 13.13). 

The polarity of the C—-OH bond, i.e., the basicity of the carbinol-amine, depends on its structure, particularly on the stability of the ring system (degree of aromatic character), and the electron affinity of the substituents on nitrogen and carbon. Of course, external factors also play an important role in the equilibrium temperature, polarity of the solvent, and presence or absence of catalysts (the solvent can also act as a catalyst). 

Five-membered heteroaromatic ring compounds have been extensively studied in terms of their relative reactivity and specific physicochemical properties. In particular, it is generally accepted that the degree of aromaticity follows the order... 

Figure 46 The planar structures of the boron ring systems, borazine (71) and boroxine (72), with nonexistent aromaticities and of triphosphatriborin (73) with an appreciable degree of aromaticity. (Adapted from ref. 95.)...

The electronic structures of furan, thiophene, and selenophene, their protonated complexes, and their anions have been calculated by the extended Hiickel method.6 The results of these calculations have been used to determine the influence of the heteroatom on the degree of aromaticity and electron density. 

The concept of aromaticity is important in the rationalization of chemical reactivity and, by extension, in the understanding of biological properties and the prediction of technological behavior. In heteroaromatic chemistry, the degree of aromaticity is of particular importance in guiding our understanding of aromaticity. 

In carbocyclic chemistry, rather firm dividing lines usually exist between aromatic, non-aromatic, and anti-aromatic compounds, while in heterocyclic chemistry enormous variations in the extent of aromatic character are displayed.52 Furthermore, there is an enormous number of potential heterocycles as compared to carbocycles, as will be detailed in section 3 of this review. The degree of aromaticity has classically been judged qualitatively in connection with the diene character of heterocycles manifested in Diels— Alder reactions or polymerizations. In this regard for instance, furan (42) is less aromatic than benzene (43), as is isoindole (44) compared to indole (45) (Scheme 18). Therefore, a quantitative aromaticity scale would be useful. 

Nitrogen. Pyridine is one of the most important heterocycles. The aromaticity of pyridine was intensively connected to structural considerations and chemical behavior. The relative difference between the aromaticity of benzene and pyridine is controversial generally calculations give similar orders of magnitude and differences depend on the criterion of aromaticity considered and the mode of calculation used. A comprehensive review on the theoretical aspects in connection with the aromaticity of pyridine was published.191 Pyridine is about as aromatic as benzene according to theoretical calculations and to experimental data, while quinoline is about as aromatic as naphthalene and more aromatic than isoquinoline.192193 The degrees of aromaticity of pyridine derivatives strongly depend on their substituents. 

There are early reviews discussing the chemistry and properties of pseudoazulenes.311312 Unsubstituted pseudoazulenes are rather unstable, but benzocon-densation and substituents stabilize the molecule, for the latter in agreement with the electronegativities of the heteroatoms. No Diels—Alder reactions occur between pseudoazulenes and dienes, a fact which is consistent with a certain degree of aromaticity. A pseudoazulene with a pyranic ring was formed from benzyl and cyclopentadiene with sodium methox-ide.313... 

The phosphorus analogue of pyrrole, phosphole, has a degree of aromatic character, according to molecular orbital calculations and nmr spectra (Brown, 1962 Chuchman et al., 1971). 1-Methyl-phosphole has a p/fg-value of 0-5 (Quin et al., 1969), much higher than that of pyrrole. It polymerizes rapidly in aqueous acid. The site of protonation of 1,2,5-triphenylphosphole is phosphorus according to the infrared spectra of some of its stable salts (Chuchman et al., 1971). 

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