Page 744 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

8.2.2. Benzene 727

[6]Annulene is benzene. Its properties are so familiar to students of organic SECTION 8.2
chemistry that not much need be said here. It is the parent compound of a vast series The Annulenes
of derivatives. As we indicated in Section 8.2.1, benzene exhibits all the properties
associated with aromaticity. It shows exceptional thermodynamic stability and its
perfectly hexagonal structure sets the standard for bond uniformity. Benzene is much
less reactive than conjugated polyenes toward electrophiles, which is in line with the
energy of the HOMO of benzene being lower than that for the HOMO of a conjugated
polyene. Benzene also exhibits NMR and magnetic susceptibility criteria consistent
with aromaticity.

8.2.3. 1,3,5,7-Cyclooctatetraene

57
The next higher annulene, cyclooctatetraene, is nonaromatic. It is readily isolable
and the chemical reactivity is normal for a polyene. X-ray structure determination
19
shows that the molecule is tub-shaped, and therefore is not a planar system to which
the Hückel rule applies. The bond lengths around the ring alternate as expected for a
polyene. The C=C bonds are 1.33 Å, whereas the C−C bonds are 1.462 Å in length. 58
Thermodynamic analysis provides no evidence of any special stability. 59 There have
been both experimental and theoretical studies aimed at estimating the relative stability
60
of the planar form of cyclooctatetraene. HF/6-31G* calculations find the completely
delocalized D structure to be about 4.1 kcal higher in energy than the conjugated
8h
planar D 4h structure, suggesting that delocalization leads to destabilization. 61 Similar
results are obtained using MP2/CASSCF calculations. 62

D 2d D 4d D 8d
Relative 0 10.6 kcal/mol 14.7 kcal/mol
Energy

These two energies are, respectively, comparable to the experimental activation
energies for conformation inversion of the tub conformer and bond shifting, suggesting
that the two planar structures might represent the transition states for those processes.
The E have been measured for several substituted cyclooctatetraenes. According to
a

57 G. Schroeder, Cyclooctatetraene, Verlag Chemie, Weinheim, 1965; G. I. Fray and R. G. Saxton, The
Chemistry of Cyclooctatetraene and Its Derivatives, Cambridge University Press, Cambridge, 1978.
58
M. Traetteberg, Acta Chem. Scand., 20, 1724 (1966).
59
R. B. Turner, B. J. Mallon, M. Tichy, W. v. E. Doering, W. Roth, and G. Schroeder, J. Am. Chem.
Soc., 95, 8605 (1973).
60 L. A. Paquette, Acc. Chem. Res., 26, 57 (1993).
61 D. A. Hrovat and W. T. Borden, J. Am. Chem. Soc., 114, 5879 (1992); P. Politzer, J. S. Murray, and
J. M. Seminario, Int. J. Quantum Chem., 50, 273 (1994).
62
J. L. Andres, D. Castano, A. Morreale, R. Palmeiro, and R. Gomperts, J. Chem. Phys., 108, 203 (1998).