Naphthalene Kekule structures

Molecular orbital calculations snggest that the jr electrons in naphthalene are delocalized over the two rings and this results in substantial stabilization. These molecules are planar, and all p orbitals are suitably aligned for overlap to form n bonding molecular orbitals. Although we can draw Kekule structures for these compounds, it is strictly incorrect to use the circle in hexagon notation since the circle represents six jr electrons. Naphthalene has 10 carbons, and therefore 10 jr electrons, and anthracene has 14 jr electrons. The circle notation suggests 12 or... 

The following bonds occur in benzenoid hydrocarbons. Consider first the endocyclic carbon-carbon bonds, namely, those found in benzene, with = 115.39 (or Sqq = 124.84) kcal/mol, which—in a sketchy way—are some kind of averages between a single and a double sp -sp bond. (Their number is double that of the number of double bonds that can be written in classical Kekule structures, e.g., 2 X 5 in naphthalene, 2 x 7 in anthracene.) But in polynuclear benzenoid structures there are not twice as many averages as there are Kekule double bonds. Hence, consider the extra single C sp )—C sp ) bonds like the one found in naphthalene, or the two extra single bonds found in anthracene. The appropriate bond energy formulas are... 

A theoretical understanding of furan and its congeners followed in the wake of the progress with the benzene problem, but for the five-membered heterocyclic compounds Bamberger s centric formula was always the most reasonable one, as Kekule structures could not be written at all. It is not now clear to what extent the heterocycles were really regarded as benzene analogues, but a remark made by Hantzsch, that coumarone is the furfurane of the naphthalene series was certainly percipient. Furan is variously said to be aromatic, superaromatic or not aromatic at all, for the debate continues. 

The frequency exaltation of the Kekule-type b2u modes of the electronically excited l B state is not limited to benzene and its derivatives. A similar observation was made by Michl and co-workers238 for [ 14]-annulene, who explained the phenomenon in terms of the avoided crossing of the Kekule structures similar to the above. Other hydrocarbons such as naphthalene, anthracene, etc. have been reported to exhibit the same phenomenon. Thus, in naphthalene,239240 the Kekule-type mode undergoes a frequency exaltation of 189 cm 1 in the 11 B2u state relative to the ground state. In anthracene, two Kekule-type modes exist. One was assigned and undergoes an upshift of 231 cm-1.241-243 The second anthracene mode has not been definitely assigned yet. It is calculated to be exalted by 96 cm-1.243... 

The arguments presented in the case of benzene can be extended to other acenes, provided the excited state and the ground state may be described as Kekule pairs along a suitable coordinate. In the case of naphthalene, a SCVB computation244 showed that the ground state is very well represented by the three Kekule structures. Taking the naphthalene computation as a proof of principle, we have adopted the Kekule model for higher acenes.3... 

The three classical Kekule structures (already alluded to in section III.E) of naphthalene are shown in Scheme 36a. Two of them are designated as Ki and K2 and represent the annulenic resonance along the perimeter of the naphthalene, while the third one, Kc, has a double bond in the center and transforms as the totally symmetric irreducible representation, Ag of the Dzh group. The Ki and K2 structures are mutually interchangeable by the i, C2, and ov symmetry operations of the point group, much as in the case of benzene. An in-phase combination transforms, therefore, as Ag, whereas an out-of-phase one transforms as B2u. These symmetry adapted wave func-... 

Scheme 36. (a) Kekule Structures for Naphthalene, (b) A VB Mixing Diagram Which Shows the Twin States Generated from the Ag and B2u Combinations of the Kekule Structures... 

FIGURE 7.6 (a) Kekule structures of naphthalene, (b) The corresponding VB mixing diagram, (c) The b2u mode that interchanges Ki and K, in the VBSCD. 

Naphthalene and anthracene are archetypes of the even and odd members of the polyacene series. In each subseries, one can start by classifying the classical Kekule structures by using the symmetry operations i, C2, and point group. Then one can form symmetry-adapted linear combinations of the mutually transformable Kekule structures and deduce their bonding characteristics. Finally, these 1 Ag and 1 B2u symmetry-adapted combinations are allowed to mix and form the states of interest, the ground and first covalent excited states (16). 

For the hydrocarbons so far considered, which consist of benzene rings connected linearly, the number of Kekul structures is one more than the number of rings. Thus in benzene it is two, naphthalene three, anthracene four, naphthacene five and dibenzanthracene six. The number of structures with elongated tt bonds, however, increases considerably as the number of benzene rings in the molecule is increased and it is this fact that is responsible for the gradual increase in reactivity with the size of the molecule. 

Figure 15 Labeled Kekule structure K, S3 structure of benzene, and D3j structure of naphthalene...

Figure 3.8. Semilattice of the mesomeric Kekul structure of naphthalene.

Write the Kekule structures of the molecules (Fig. 8.6) of naphthalene (three structures), anthracene (four structures), and phenanthrene (five structures). 

According to resonance theory, a molecule of naphthalene can be considered to be a hybrid of three Kekule structures. One of these Kekule structures, the most important one, is shown in Fig. 14.15. There are two carbon atoms in naphthalene (C4a and C8a) that are common to both rings. These two atoms are said to be at the points of ringfusion. They direct all of their bonds toward other carbon atoms and do not bear hydrogen atoms. 

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. 

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