Azulene, dipole moment structure

Azulene, dipole moment of. 54 electrostatic potential map of, 541 structure of, 533... 

Azulene does have an appreciable dipole moment (0.8 The essentially single-bond nature of the shared bond indicates, however, that the conjugation is principally around the periphery of the molecule. Several MO calculations have been applied to azulene. At the MNDO and STO-3G levels, structures with considerable bond alternation are found as the minimum-energy structures. Calculations which include electron correlation effects give a delocalized n system as the minimum-energy structure. ... 

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. 

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. 

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

The dipole moment of azulene is small (0.80 D) [50] this suggests that the contribution of dipolar forms to the overall structure is small, but nonetheless real. The direction of the dipole is confirmed by dipole moment measurements on 2-halogeno- and 2-cyano-azulenes [51]. Calculations agree that the five- and seven--membered rings of azulene should carry, respectively, negative and positive charges. 

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. 

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

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