Page 753 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

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CHAPTER 8
Aromaticity

Kekulene

Fullerene, C , is a spherical form of carbon that is produced by processes
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such as laser vaporization of graphite. 111 The structure consists of hexagons and
pentagons, corresponding to the pattern of a soccer ball. There is bond length
variation with the bonds shared by the hexagonal rings being shorter (1.40 ± 0.01 Å)
than those of the pentagons (1.46 ± 0.01 Å). Unlike benzene, with its two-Kekule
structure, there is only one valence bond structure for C . It has double bonds at
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all hexagon-hexagon edges and single bonds at the pentagonal edges. An isodesmic
energy computation suggests that the system is substantially less stable than for
benzene on an atom-by-atom comparison. 112 Calculated chemical shift parameters
suggest that the five-membered rings are antiaromatic, whereas the hexagonal rings
are aromatic. 113 Thus it appears that fullerene is a delocalized molecule, but with both
stabilizing and destabilizing components, which are partially compensating in terms of
stabilization energy.
It was pointed out that a different array of atomic orbitals might be conceived of
in large conjugated rings. The array, called a Mobius twist, results in there being one
point in the ring at which the atomic orbitals have a phase reversal (node). 114

If the ring were sufficiently large that the twist between individual orbitals was small,
such a system would not necessarily be less stable than the normal array of atomic
orbitals. This same analysis points out that in such an array the Hückel rule is reversed
and aromaticity is predicted for the 4n -electron systems.

111 H. W. Kroto, J. P. Heath, S. C. O’Brien, R. F. Curl, and R. E. Smalley, Nature, 318, 162 (1985).
112
P. W. Fowler, D. J. Collins, and S. J. Austin, J. Chem. Soc., Perkin Trans. 2, 275 (1993).
113 P. v. R. Schleyer, C. Maerker, A. Dransfeld, H. Jiao, and N. J. R. van Eikema Hommes, J. Am. Chem.
Soc., 118, 6317 (1996).
114
E. Helibronner, Tetrahedron Lett., 1923 (1964).