Azulene resonance structures

Azulene, an isomer of naphthalene, lias a remarkably large dipole moment for a hydrocarbon (/i = 1.0 D). Explain, using resonance structures. 

Azulene can be written as fused cyclopentadiene and cycloheptatriene rings, neither of which alone is aromatic. However, some of its resonance structures have a fused cyclopentadienyl anion and cycloheptatrienyl cation, which accounts for its aromaticity and its dipole moment of 1.0 D. 

A-5. Azulene, shown in the following structure, is highly polar. Draw a dipolar resonance structure to explain this fact. 

Calicenc, like azulene (Problem 15.36), has an unusually large dipole moment fori hydrocarbon. Explain, using resonance structures. 

Azulene has an appreciable dipole moment. Write resonance structures for azulene that PRACTICE PROBLEM 14.12 explain this dipole moment and that help explain its aromaticity. 

In some cases, a resonance structure is required to see an aromatic system. The increased stability associated with an aromatic system is found for the structure, although the compounds do not appear aromatic unless the resonance structure is considered. Azulene, which can be drawn as a cyclopentadienyl anion fused to a cycloheptatriene cation, and cyclopropenone, which can be written as possessing a cyclopropenyl cation, are two examples (see margin). 

Figure 4.2 (a) Resonance structures of azulene. (b) HCickel frontier molecular orbitals of azulene. (Reproduced with permission [5], Copyright 1988, American Chemical Society.)... 

When considering polycyclic systems, it is necessary to delete all e ntial single or essential double bonds. These bonds can be readily identified since they are the same in all classical resonance structures of the molecule. Consider azulene the classical resonance structures are shown in Fig. 2.11. Notice that the central bond is always single and is, therefore, omitted, leaving a mono-cyclic system with ten carbon atoms, i.e. an aromatic system. 

Therefore, this resonance structure contributes significant character to the overall resonance hybrid, which gives the azulene a considerable dipole moment. 

The crucial structural feature which underlies the aromatic character of benzenoid compounds is of course the cyclic delocalised system of six n-electrons. Other carbocyclic systems similarly possessing this aromatic sextet of electrons include, for example, the ion C5Hf formed from cyclopentadiene under basic conditions. The cyclopentadienide anion is centrosymmetrical and strongly resonance stabilised, and is usually represented as in (7). The analogous cycloheptatrienylium (tropylium) cation (8), with an aromatic sextet delocalised over a symmetrical seven-membered ring, is also demonstrably aromatic in character. The stable, condensed, bicyclic hydrocarbon azulene (Ci0H8) possesses marked aromatic character it is usually represented by the covalent structure (9). The fact that the molecule has a finite dipole moment, however, suggests that the ionic form (10) [a combination of (7) and (8)] must contribute to the overall hybrid structure. 

The only satisfactory approach to this problem at present involves the use of perturbations methods. Attempts to calculate resonance energies indirectly, by comparing calculated total energies of a compound with that of a single classical structure for it, have proved ineffective. Thus calculations of this kind have failed to account for the special stability of monocyclic compounds containing 4n + 2 electrons (Hiickel s rule), and they predict that all the compounds XVII-XXIII should be aromatic whereas the only one that is aromatic is azulene (XXII). 

Tropone, 1 (a), and azulene, II (a), shown in Fig. 4 have resonance energies, of 12 and 28 kcal/mole, respectively. The theoretical implication of a stable system being achieved by six r-electrons, is that the structures are closer to those shown in I (6) and II (5) certainly this is in keeping with the observed physical and chemical properties. 

Azulene is one of the few completely conjugated nonbenzenoid hydrocarbons that appears to have appreciable aromatic stabilization. There is some divergence on this point between the SCF-MO and HMO treatments. The latter estimates a resonance energy about half that for the isomeric naphthalene, whereas the SCF-MO method assigns a resonance energy which is only about one seventh that of naphthalene. The parent hydrocarbon and many of its derivatives have been well characterized and are stable compounds. The structure of azulene itself has been... 

Fig. 11-32 illustrates the difficulty in studying CPs in the solid state. Here the low-temperature, C MAS spectrum of undoped poly(azulene) is shown together with that of a monomer derivative [441]. The broad resonance for the polymer bespeaks delocalized charge or disordered polymer structure, but does not convey much additional information. 

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