Phenanthrene Kekule structures

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. 

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

We first consider the numeric Kekule structures of cata-condensed benzenoids. As an example, we consider dibenzo[b, ]phenanthrene. In Fig. 9.14 we give geometric Kekule stmctures of dibenzo[b,g]phenanthrene and in Fig. 9.15 their numeric counterparts. Kekule structures of dibenzo[b,g]phenanthrene can be generated in... 

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 the first column of Figure 18 we show the five Kekule valence structures of phenanthrene, and in each row we depict all its conjugated circuits, which include also combinations of disjoint conjugated circuits. The reason for considering disjoint conjugated circuits follows from an observation that otherwise some Kekule valence structures would have more conjugated circuits than others. Thus, while the first four Kekule structures of phenanthrene have three conjugated circuits each, if we do not count the disjoint combinations shown in the last column of the first four rows, the last Kekule structure would have... 

It suffices to assign C=C type to any of the vertical CC bonds in such molecules, and the bond types of all other CC bonds will be determined. Of the five Kekule valence structures in phenanthrene shown in Figure 18, only the last (anti-Fries) structure has df = 1. The remaining four Kekule structures of phenanthrene have df = 2, because a selection of C=C bond in one of the terminal benzene rings can in no way influence the selection of C=C bonds in the other terminal ring. In Figure 20 we have illustrated some Kekule valence structures having different d/values for a selection of small benzenoid hydrocarbons. 

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... 

Write the five Kekule-type resonance structures of phenanthrene and show how these structures can account for the fact that phenanthrene, unlike benzene, adds bromine, but only across the 9,10-positions. 

Exercise 22-31 Draw the Kekule-type valence-bond structures for napthalene, anthracene, and phenanthrene. Estimate the percentage of double-bond character for the 9,10 bond of phenanthrene, assuming that each of the valence-bond structures contributes equally to the hybrid structure. 

Fi g. 5 The Kekule valence structures of phenanthrene and their innate degrees of... 

Because Clar structures can be viewed as a superpositions of selected Kekule valence structures we will briefly examine pair-wise superposition of the five Kekule valence structures of phenanthrene (shown in Fig. 5). In all we can construct ten combinations illustrated in Fig. 8. First,... 

All pair-wise combinations of the five Kekule valence structures of phenanthrene. 

---