Page 769 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

752 The ring current in benzocyclobutadiene has been analyzed in detail. 178 The main
ring current is associated with the four-membered ring and is paramagnetic. This is
CHAPTER 8
consistent with the calculated NICS values, which are −2 5 for the six-membered
Aromaticity 179
ring and 22.5 for the four-membered ring. The fusion of the cyclobutadiene ring
to benzene greatly diminishes the aromatic character of the benzenoid ring. The
implication of a nonaromatic structure is that the combination of ring strain and the
antiaromaticity associated with the four-membered ring results in a localized system. 180
Azulene is one of the few nonbenzenoid hydrocarbons that appear to have appre-
ciable aromatic stabilization. There is some divergence on this point between the
SCF-MO and HMO’ results in Scheme 8.2. The latter estimates a resonance energy
about half that for the isomeric naphthalene, whereas the SCF-MO method assigns
a resonance energy that is only about one-seventh that of naphthalene. Naphthalene
is thermodynamically more stable than azulene by about 38.5 kcal/mol. Molecular
mechanics calculations attribute about 12.5 kcal/mol of the difference to strain and
about 26 kcal/mol to greater resonance stabilization of naphthalene. 181 Based on heats
of hydrogenation, the stabilization energy of azulene is about 16 kcal/mol. 182 The
parent hydrocarbon and many of its derivatives are well-characterized compounds with
considerable stability. The structure of azulene has been determined by both X-ray
crystallography and electron-diffraction measurements. 183 The peripheral bond lengths
are in the aromatic range and show no regular alternation. The bond shared by the two
rings is significantly longer, indicating that it has predominantly single-bond character,
which indicates that the conjugated system more closely resembles [10]annulene than
naphthalene. Theoretical calculations indicate that the molecule has C 2v symmetry,
suggesting delocalization of the electrons. 184

1.391 1.400 1.383 1.406
1.392 1.403
1.498 1.501
1.394 1.399
1.398 1.418
electron diffraction
azulene X-ray bond lengths
bond lengths
An interesting structural question involves the contribution of a dipolar structure that
pictures the molecule as the fusion of a cyclopentadienide anion and a cycloheptatrienyl
cation.

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180
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182
W. R. Roth, M. Boehm, H. W. Lennartz, and E. Vogel, Angew. Chem. Int. Ed. Engl., 22, 1007 (1983).
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184
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