Anthracene 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... 

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... 

Scheme 38. (a) Four Kekule Structures for Anthracene and (b) Their Symmetry Adapted Combinations, (c) Generation of the Twin States 0 o, I ) from These Combinations... 

Scheme 39. Kekule Modes that Interchange the Benzenic (B) and Annulenic (A) Kekule Structures of Anthracene (Ref 3)a...

Figure 7.7a shows the four classical Kekule structures of anthracene (16,19,24). Two of the structures involve resonance in the central benzenic ring and are therefore labeled as KiB and K2B. The other two involve annulenic resonance along the molecule perimeter, and are labeled accordingly as K1A and K2a- The structures of the types A and B form two symmetry subsets, and within each subset, the two structures are mutually transformable by the D2h symmetry operations (i, C2, and ov). Therefore, as shown in Fig. 7.7b, within each subset there will be a positive combination that transforms as Ag and a negative one that transforms as B2u. 

FIGURE 7.7 (a) Kekule structures of anthracene, (b) Symmetry adapted combinations... 

Since the ground state and the 1 IT, excited state are made from two sets of Kekule structures that are interchanged by two b2u modes, one will expect to find in the spectrum of the B2u state two b2u modes with exalted frequencies as shown in Fig. 7.8. As can be seen in the figure, one of these modes is benzenic, the other is annulenic. These two exalted modes are indeed observed in the B2u spectrum of anthracene (16,19). The bonding features of the B2u state are expected to involve spin pairing between nonconsecutive carbons... 

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). 

In practice, the valence bond picture has probably exerted more influence on how chemists actually think than the HMO picture. However most early applications were primarily qualitative in nature. This qualitative VB picture can be summarized under die name of resonance theory [10]. The basic concept is that in general the more ways one has of arranging the spin pairing in the VB wave function, the more stable the molecule is likely to be. Thus, VB theory predicts that phenanthrene with 14 carbon atoms and 5 Kekule structures should be more stable than anthracene with 14 carbon atoms but just 4 Kekule structures, in complete accord with the experimental evidence. It also predicts that benzenoid hydrocarbons with no Kekule structures should be unstable and highly reactive, and in fact no such compounds are knowa Extensions of this qualitative picture appear, for example, in Clar s ideas of resonant sextets [11], which seem to be very powerful in rationalizing much of the chemistry of benzenoid aromatic hydrocarbons. The early ascendancy of HMO theory was thus largely based on the ease with which it could be used for quantitative computations rather than on any inherent superiority of its fundamental assumptions. 

Fig. 2. Kekule structures and generalized Clar structures of benzo[a]anthracene...

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. 

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

F. 8.5 Upper row the four Keknle (geometric) structures for anthracene with their numerical (algebraic) counterparts inscribed in each ring. Each of the four gemnetric Kekule stmctures has a unique partition of tt-electrons ot algebraic Kekule structure 6,4,4 5,5,4 4,5,5 and 4,4,6. Lower row the EC values for the marginal and central rings of anthracene... 

Kekule structures for the open chain graph of anthracene. 

Now we insert the line corresponding to the C13-C14 cr bond of anthracene, then delete this line and its vertices to produce the two fragments shown on the top right in Figure 4.71. Both of these fragments have only one Kekule structure, so the SC for the anthracene molecule is34-lxl=4. That result agrees with the number of Kekule resonance forms for anthracene determined by inspection (Table 4.4). 

Assuming that the resonance energy is proportional to (//— 1) where n is the number of possible Kekule structures, calculate the resonance eneiigies of naphthalene, anthracene, and phenanthrene from the value for benzene. The observed values are 314, 440, and 459 kJ mole, respectively. 

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 ... 

In some cases finding the df is simple. It is not difficult to see that all Kekule structures of linearly fused benzenoid hydrocarbons — naphthalene, anthracene, tetracene, pentacene, etc. — have df=... 

Figure 134. Kekule structures of naphthalene, anthracene, and phenanthrene, showing the count of jr-elec-trons involved in each ring.

Notice that anthracene cannot be represented by any single Lewis structure m which all three rings correspond to Kekule formulations of benzene but phenanthrene can... 

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