Aromatic character, definition

These molecules must be regarded as essentially nonaromatic, although with some aromatic character. Tropolones readily undergo aromatic substitution, emphasizing that the old and the new definitions of aromaticity are not always parallel. In sharp contrast to 44, cyclopentadienone (46) has been isolated only in an argon matrix below 38 Above this temperature, it dimerizes. Many earlier attempts to prepare it were unsuccessful. As in 44, the electronegative oxygen atom draws electron to itself, but in this case it leaves only four electrons and the molecule is unstable. Some derivatives of 46 have been prepared. ... 

Much more stable are the 2- or 4-oxides, i.e., a-pyrones (unsaturated lactones) 20 and y-pyrones, which could, in principle, have aromatic character owing to their zwitterionic resonance structures (20A-20C). However, although protonated pyrones are definitely aromatic, the neutral compounds appear to have very little aromaticity. Bird s aromaticity index I6 for pyrylium is only 65.8% in comparison with benzene, whereas for 4-pyrone it is 37.2% and for 2-pyrone it is only 32.9%, as seen in Table 4 [22], In agreement with Table 2, the ring 0-(C = O) bond in 20A is a type X-Z bond, whereas the ring O = (C-O ) bond in the two other resonance formulas is a Y-Y bond. 

There has been no work published assessing the aromatic character of 1,2,3-thiadiazole. From the Huckel definition of aromaticity, (4n + 2) ir-electrons in a ring constitute an aromatic compound and by this criterion 1,2,3-thiadiazoles should be considered aromatic compounds. Chemical shifts for 1,2,3-thiadiazoles in both 13C and 1H NMR, which are related to the electronic environment, are in accord with an aromatic heterocyclic ring. 

The described definitions have been widely used to characterize aromaticity. However, the correlation between these criteria and aromatic (or antiaromatic) nature is by no means simple. Even more so, the aromatic character is, by definition, a relative property, namely, one can assign aromaticity only by referring to model compounds. Thus, even when the number of 71-electrons calls for the existence of aromatic character, its extent cannot be predicted. Similarly the relationship between magnetic susceptibility of a polycycle and its conjugative stabilization is highly complex 105,106). 

The term aromaticity or aromatic character is used in organic chemistry to indicate an extremely high thermodynamic ground-state stability, as well as high kinetic stability, which may be observed in some cyclic conjugated unsaturated compounds in comparison with appropriate non-cyclic unsaturated reference compounds i 4K This definition is vague because aromaticity denotes an experimental excess property whose magnitude depends on a more or less arbitrary selection of the reference compound. 

The fact that aromatic compounds differ from unsaturated aliphatic compounds was recognized as early as the nineteenth century. However, chemists are still unable to agree on a suitable definition of aromatic character <(68MI 209-01). Qualitatively, the concept of aromaticity is quite clear <74AHC(17)255, b-85MI 209-02). A compound is considered aromatic if it ... 

Michal Ksawery Cyrailski was born in Warsaw, Poland (1970). In 1994 received his M.Sc. degree and in 1999 his Ph.D. degree, both at the Chemistry Department of Warsaw University. In 1997 he was a holder of the Foundation for Polish Science grant in 1998 he received the distinction of the Kemula Reward (by the Polish Chemical Society) and in 1999 the Kolos Reward for the best Ph.D. work (by the Department of Chemistry of Warsaw University). In 1998-2000 he was a secretary of the Executive Committee of the Polish Chemical Society (elected). His main scientific interests are the structural chemistry of small organic compounds, crystallochemistry, molecular modeling (ab initio), structural aspects of aromatic character of r-electron systems, and definition of aromaticity. So far, he has published over 30 original papers and 5 reviews and presented 5 lectures and over 30 poster and oral communications. His hobby is classical music, especially by J. S. Bach, and singing in a professional choir. 

All (4A7 -I- 2) annulenes clearly exhibit aromatic character, whereas 4 A7 annulenes are definitely antiaromatic, with negative HOMA. 

An earlier review [4] says "By gaining in generality aromatic character grows steadily more diffuse and qualitative until we are ultimately left with chemical intuition as the only test of its validity. On the other hand attempts to reduce aromaticity to an observable seem to result in definitions that are too narrow to be acceptable to the chemist". It goes on to add that this is a typical state of affairs in chemistry, rather than an exception, and expresses hope that a time will come "when we acquire the intellectual tools to cope with the problem of conqilexity in chemistry more directly". 

On the other hand, in aromatic compounds, particularly in benzene, a decrease is apparent in the molar refraction. This depression is specific to the aromatic character and is consistent with the fact that, on account of the ring arrangement of the three double bonds within the nuclear framework, a definite distribution of negative charge density occurs. The same consequence was deduced from quantum-mechanical considerations by the far-reaching studies of Hiickel, Pauling and Slater. It... 

M. Randic, rc-Electron currents in polycyclic conjugated hydrocarbons of decreasing aromatic character and novel structural definition of aromaticity. Open Org. Chem. J. 5 (Suppl. 1-M2) (2011) 11-26. 

Much effort has been directed at asphaltenes flocculation and precipitation in petroleum crude mixtures that is a significant problem in oil production, transmission and processing facilities. Pressure, temperature, the chemical composition of the oil and the amount of dissolved gases affect this undesired phenomenon. In general, the prediction of asphaltenes precipitation is very difficult and suffers from the definition of an asphaltene. Usually, asphaltenes are defined as the part of the crude oil that is soluble in methylbenzene and benzene but insoluble in pentane or heptane. Asphaltenes consist of many thousands of species, differing in size and chemical structure. The aromatic character of the asphaltenes and their content of heteroatoms influence their solubility in different solvents and the tendency to flocculate. In most of the calculations of the phase equilibria the pseudo-component method has been... 

We will take a quick look here at reactions. We will first introduce a general reaction that preserves the aromatic character of benzene. It is risky even to suggest that molecules have desires, but the imperative in benzene chemistry definitely is, Preserve the Aromatic Sextet A less flamboyant way of saying this is to point out that thermodynamics will surely favor very stable molecules, and therefore benzene chemistry is likely to lead to more very low energy aromatic compounds. However, occasionally there are usefiil reactions in which the aromatic stabilization is lost, and we will see one of them in Section 13.11b. 

The highly polar carbamate pesticides cannot always be analysed by GC-MS (Figures 3.95-3.97). The low thermal stability leads to decomposition even in the injector. Substances with definite aromatic character, however, form stable intense molecular ions. 

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