Aromatic compound Kekule ring structure

The resonance-delocalized picture explains most of the structural properties of benzene and its derivatives—the benzenoid aromatic compounds. Because the pi bonds are delocalized over the ring, we often inscribe a circle in the hexagon rather than draw three localized double bonds. This representation helps us remember there are no localized single or double bonds, and it prevents us from trying to draw supposedly different isomers that differ only in the placement of double bonds in the ring. We often use Kekule structures in drawing reaction mechanisms, however, to show the movement of individual pairs of electrons. 

Naphthalene Naphthalene (C10H8) is the simplest fused aromatic compound, con- Hydrocarbons sisting of two fused benzene rings. We represent naphthalene by using one of the three Kekule resonance structures or using the circle notation for the aromatic rings. 

Figure 5.3 Examples of organic molecules containing tt bonds. Note that benzene rings can be drawn showing three tt bonds (the Kekule structure) or with a circle inside the ring, as has been done for ethylbenzene, to more accurately depict the delocalized nature of the tt electrons in aromatic compounds.

As you draw the structure of aromatic compounds, remember that only one hydrogen atom or group can be attached to a particular position on the benzene ring. For example, compounds (A) and (B) below exist, but (C) does not. Examination of the Kekule structure... 

In Chapter 14 we shall study in detail a group of unsaturated cyclic hydrocarbons known as aromatic compounds. The compound known as benzene is the prototypical aromatic compound. Benzene can be written as a six-membered ring with alternating single and double bonds, called a Kekule structure after August Kekule, who first conceived of this representation ... 

When molecular orbital calculations are carried out for naphthalene using the model shown in Fig. 14.16, the results of the calculations correlate well with our experimental knowledge of naphthalene. The calculations indicate that delocalization of the 10 electrons over the two rings produces a structure with considerably lower energy than that calculated for any individual Kekule structure. Naphthalene, consequently, has a substantial resonance energy. Based on what we know about benzene, moreover, naphthalene s tendency to react by substitution rather than addition and to show other properties associated with aromatic compounds is understandable. 

Cyclic compounds other than benzene can, however, possess aromatic or benzenoid properties. This arises when they are planar and possess some double bonds which enable their formulae to be expressed in alternative Kekule-type structures. In such compounds an overlap of p orbitals occurs between adjacent atoms, and this allows the n electrons to become delocalised and form a continuous ring of electron density (Figure 6.21). An aromatic compound of this kind will sustain a magnetically induced ring current, and the bond lengths are all equivalent, lying between single and double bond values. Benzene with 6 n electrons was the first of these aromatic compounds to be encountered and seriously studied by chemists. 

Benzene (CeHg), the parent compound of the aromatic hydrocarbons, was discovered by Michael Faraday in 1826. Over the next 40 years, chemists were preoccupied with determining its molecular structure. Despite the small number of atoms in the molecule, there are quite a few ways to represent the structure of benzene without violating the tetravalency of carbon. Most of the proposed structures were rejected, however, because they did not explain the known properties of benzene. Finally, in 1865, August Kekule deduced that the benzene molecule could be best represented by a ring structure—a cyclic compound consisting of six carbon atoms ... 

Occasionally, one comes across benzenoid hydrocarbons with several Kekule valence structures that have the same number of benzene rings with alternating double and single bonds however, some of those structures are distinct, that is, symmetry non-equivalent. Hence, such Kekule valence structures need not be equally important for the aromaticity of these compounds. In order to find out how important individual Kekuld valence structures are, we propose a model in which the relative weight of Kekuld valence structures is given by their contribution to molecular resonance energy (RE). 

Clar proposed that the maximum number of localized aromatic sextets (and thus the number of Kekule structures) that can be drawn for benzenoid hydrocarbons correlates well with several properties of the compounds. For example, phenanthrene contains two localized sextets (five Kekule resonance structures), while its isomer anthracene has only one localized sextet (four Kekule resonance structures), and so might be considered to be more aromatic . Of the 4-ring benzenoid isomers, naphthacene has the fewest sextets (one), triphenylene has the most (three), and the others have two apiece. Generally, the cata-condensed species which have more phenanthrene subunits, and thus have greater angularity , also have more localized Clar sextets. How well does the Clar model correlate with the enthalpies of formation ... 

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