Electronic distribution aromaticity

In contrast, 2- and 4-pyrones are considered to have relatively little aromatic character. Whereas in an analogous nitrogen series 4-pyridone 5.23 has significant aromatic character (mesomeric representation 5.23a making a considerable contribution to the overall electronic distribution), aromatic mesomeric representation 9.3a makes less of a contribution to the overall electronic structure of 4-pyrone. As with furan, the higher electronegativity of oxygen leads to heterocycles of little aromaticity in cases where delocalisation of electron density from the heteroatom is a prerequisite for that aromaticity. 

The TT-inductive effect describes how an inductive substituent might selectively influence the electron distribution at the o- and -positions of the aromatic nucleus. A familiar example is represented by the... 

Turning now to electrophilic aromatic substitution in (trifluoromethyl)benzene we con sider the electronic properties of a trifluoromethyl group Because of their high elec tronegativity the three fluorine atoms polarize the electron distribution m their ct bonds to carbon so that carbon bears a partial positive charge... 

The origin of cyclopropenone chemistry goes back to the successful preparation of stable derivatives of the cyclopropenium cation <5 3), the first member of a series of Huckel-aromatic monocyclic carbo-cations possessing a delocalized system of (4n + 2)-7r-electrons. This experimental confirmation of LCAO-MO theory stimulated efforts to prepare other species formally related to cyclopropenium cation by a simple resonance description of electron distribution, namely cyclopropenone 7 and methylene cyclopropene (triafulvene) 8 ... 

In a pericyclic reaction, the electron density is spread among the bonds involved in the rearrangement (the reason for aromatic TSs). On the other hand, pseudopericyclic reactions are characterized by electron accumulations and depletions on different atoms. Hence, the electron distributions in the TSs are not uniform for the bonds involved in the rearrangement. Recently some of us [121,122] showed that since the electron localization function (ELF), which measures the excess of kinetic energy density due to the Pauli repulsion, accounts for the electron distribution, we could expect connected (delocalized) pictures of bonds in pericyclic reactions, while pseudopericyclic reactions would give rise to disconnected (localized) pictures. Thus, ELF proves to be a valuable tool to differentiate between both reaction mechanisms. 

Energetic attempts are being made at this time by Coulson, Daudel, Pullman and others to refine the theory of electron distribution in polycyclic aromatic systems, but it is not clear to the writer that the best compromise between simplicity and accuracy has yet been secured. 100... 

As with the methylbenzenes the methyl derivatives of condensed aromatic substances show an increase in basicity compared to the unsubstituted compound. Thus, the methyl groups exert a very profound influence on the electron distribution. The extension of these considerations by theoretical methods will be discussed in Section V. 

In the preceding chapters we have seen how new bonds may be formed between nucleophilic reagents and various substrates that have electrophilic centres, the latter typically arising as a result of uneven electron distribution in the molecule. The nucleophile was considered to be the reactive species. In this chapter we shall consider reactions in which electrophilic reagents become bonded to substrates that are electron rich, especially those that contain multiple bonds, i.e. alkenes, alkynes, and aromatics. The jt electrons in these systems provide regions of high electron density, and electrophilic reactions feature as... 

The aromaticity of pyrrole, furan, and thiophene may also be assessed by considering the 7r-electron distribution in them (8UST163), which points to a greater aromaticity of pyrrole and thiophene relative to furan. 

It was discovered, however, that the spherical aromaticity of the icosahedral fullerenes C20, Cjq and CgQ depends on the filling of the Jt-sheUs with electrons [107]. As pointed out in Section 14.3.1 no distortion of the cage structure is expected in these fullerenes if their shells are fully filled. Closed-shell situations are realized if the fullerene contains 2(N -1-1) Jt electrons. This is closely related to the stable noble-gas configuration of atoms or atomic ions [108]. In this case the electron distribution is spherical and all angular momenta are symmetrically distributed. Correlation of the aromatic character determined by the magnetic properties is shown in Table 14.3. 

Problem 10.30 Design a table showing the structure, number of tt electrons, energy levels of tt MO s and electron distribution, and state of aromaticity of (a) cyclopropienyl cation, b) cyclopropenyl anion, (c) cyclobutadiene, (d) cyclobutadienyl dication, (c) cyclopentadienyl anion, (/) cyclopentadienyl cation, (g) benzene, (h) cycloheptatrienyl anion, (/) cyclooctatetraene, (/ ) cyclooctatetraenyl dianion. ... 

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