Phenanthrene resonance structures

There are five resonance structures of phenanthrene, one of winch is shown. Draw the other four. 

Look at the five resonance structures for phenanthrene (Problem 15.26) and predict which of its carbon-carbon bonds is shortest. 

Like phenanthrene, some PAHs have a unique Clar structure, whereas several alternative Clar structures are possible for other PAHs [21]. Thus, Clar s rule does not designate the resonance structure mainly responsible for the aromaticity of anthracene. Clar s model cannot differentiate between its outer and inner ring (Scheme 28.2). 

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. 

Phenanthrene has five total resonance structures. One is shown here. Draw the other four. Which carbon-carbon bond of phenanthrene would you predict to be the shortest ... 

The resonance theory says that if two compounds have the same set of atoms and bonds, then that compound for which more resonance structures can be drawn will be the more stable. Using this rule, which would be more stable, anthracene, phenanthrene, or benzazulene ... 

Phenanthrene is best represented as a hybrid of the five canonical forms 20-24. It has a resonance energy of 380 kJ mol and is more stable than anthracene. In four of the five resonance structures, the 9,10-bond is double and its length is about the same as an alkenic C=C bond. The numbering system for phenanthrene is shown in 20. Five different mono-substituted products are possible. 

Explain why triphenylene resembles benzene in that it does not undergo addition reactions with Br2, but phenanthrene reacts with Br2 to yield the addition product drawn. (Hint Draw resonance structures for both triphenylene and phenanthrene, and use them to determine how delocalized each iz bond is.)... 

The method just described is more difficult to apply to larger molecules. For naphthalene the HMO method leads to a 10 x 10 determinant because ten p orbitals are mixed to make the MO. For the resonance method, however, the size of the determinant is limited only by our willingness to include resonance structures. In general, there are many more resonance structures that should be considered than there are p orbitals, and a reasonable resonance treatment of naphthalene would require a 42 x 42 determinant. For anthracene and phenanthrene, the HMO determinant would be 14 x 14, but the resonance method would require solution of a 429 x 429 determinant in each case. Consideration of symmetry can reduce the phenanthrene... 

Phenanthrene has more Kekule resonance structures than does naphthalene, so we expect it to be more aromatic, and the data in Table 4.4 indicate that it does have a larger resonance energy. However, naphthalene does not undergo electrophilic addition, and phenanthrene readily undergoes electrophilic addition across the 9,10 bond. 

Polycyclic aromatic compounds such as naphthalene, anthracene, and phenanthrene give electrophilic aromatic substitution reactions. The major product is determined by the number of resonance-stabilized intermediates for attack at a given carbon and the number of fully aromatic rings (intact rings) in the resonance structures. 

Polynuclear aromatic hydrocarbons such as naphthalene, anthracene, and phenanthrene undergo electrophilic aromatic substitution reactions in the same manner as benzene. A significant difference is that there are more carbon atoms, more potential sites for substitution, and more resonance structures to consider. In naphthalene, it is important to recognize that there are only two different positions Cl and C2 (see 122). This means that Cl, C4, C5, and C8 are chemically identical and that C2, C3, C6, and C7 are chemically identical. In other words, if substitution occurs at Cl, C4, C5, and C8 as labeled in 122, only one product is formed 1-chloronaphthalene (121), which is the actual product isolated from the chlorination reaction. Chlorination of naphthalene at Cl leads to the five resonance structures shown for arenium ion intermediate 127. 

Substituents such as alkene units, alkyne units, and carbonyls can be reduced by catalytic hydrogenation. Lithium aluminum hydride reduces many heteroatom substituents, including nitrile and acid derivatives 56, 57, 104, 105, 106, 107, 108, 109. Polycyclic aromatic compounds such as naphthalene, anthracene, and phenanthrene give electrophilic aromatic substitution reactions. The major product is determined by the number of resonance-stabilized intermediates for attack at a given carbon and the number of fully aromatic rings (intact rings) in the resonance structures 59, 60, 61, 62, 63, 64, 65, 85, 104, 106, 107, 108,109,110,113,114,118. 

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 ... 

With phenanthrene, however, four of the five resonance structures keep a double bond at the labeled C s. (Only C does not.) This means that these two C s have more double bond character than other C-C bonds in phenanthrene, making them more susceptible to addition rather than substitution. 

Notice that the C9-C10 bond is a double bond in four of the five resonance structures. No other bond in phenanthrene has this feature. As such, the C9-C10 bond is expected to have the most double-bond character, and consequently, the shortest bond length. 

This treatment could be applied to anthracene and phenanthrene, with 429 linearly independent structures, and to still larger condensed systems, though not without considerable labor. It is probable that the empirical rule6 of approximate proportionality between the resonance energy and the number of benzene rings in the molecule would be substantiated. 

The condition for stabilization of the semiquinone by resonance is that the two structures IV be equivalent. This condition is satisfied for the semiquinone anion, but not for the semiquinone III itself, in which the presence of the hydrogen atom destroys the equivalence of the two structures. We thus expect the semiquinone to be stable only in the form of the anion. This is verified by experiment. Michaelis and his collaborators67 have shown that the semiquinone of phenanthrene-3-sulfonate is stable in alkaline solution as the semiquinone ion,... 

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. 

---