Page 42 - Mechanism and Theory in Organic Chemistry

Aromaticity 31

matrix at very low temperature.6 Cyclooctatetraene, though a stable compound,
does not have a planar ring; whatever stabilization we might have expected it to
gain from the delocalization 29- 30 is evidently not sufficient to cause the

molecule to abandon its tub-shaped conformation (31) for the planar structure
that would allow cyclic conjugation.
The simple resonance theory fails to explain the singular lack of effective-
ness of delocalization in cyclobutadiene and cyclooctatetraene, but we may turn
to molecular orbitals for the solution.

rr-Electron Theory and the Hiickel4n + 2 Rule
In order to construct a bonding model for a planar conjugated ring, we follow the
procedure outlined for the ally1 system in the previous section and make the
choice of sp2 hybridization on each carbon. The a framework is then constructed
from these sp2 hybrids and the hydrogen 1s orbitals, leaving a p orbital on each
carbon. We next concentrate on the interactions among these p orbitals:
Erich Hiickel developed this theory in the 1930s. He discovered that the
energies of the .rr molecular orbitals for any regular plane polygon with an even
number of atoms will fall in the pattern 32.9 A polygon with an odd number of
atoms gives the pattern 33. These patterns, a single lowest level with higher levels

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.
Energy
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(a) L. Watts, J. D. Fitzpatrick, 2nd R. Pettit, J. Amer. Chem. Soc., 87, 3253 (1965); (b) 0. L.
Chapman, C. L. McIntosh, and J. Pacansky, J. Amer. Chem. Soc., 95, 614 (1973); (c) G. Maier and
M. Schneider, Angew. Chem. Int. Ed., 10, 809 (1971). For a summary of attempts to prepare cyclo-
butadiene, see (d) M. P. Cava and M. J. Mitchell, Cyclobutadiene and Related Compoundr, Academic
Press, New York, 1967.
(a) E. Huckel, Z. Physik, 70, 204 (1931); (b) E. Huckel, Z. Physik, 76, 628 (1932); (c) E. Huckel,
Z. Electrochem., 43, 752 (1937).