Frost circles 1,3,5,7-cyclooctatetraene

Figure 2.28 Relative energies of benzene and cyclooctatetraene molecular orbitals from Frost circles...

Cyclooctatetraene is another example of a An molecule. But here we find neither the special stability of benzene, nor the special instability of cyclobutadiene. The molecule behaves like four separated, normal alkenes. Let s first examine planar cyclooctatetraene.The Frost circle again determines the relative positions of the molecular orbitals (Fig. 13.22). 

FIGURE 11 13 Frost s circle and the TT molecular orbitals of (a) square cyclobutadiene (b) ben zene and (c) planar cyclooctatetraene... 

Annulene is a 4n -electron system, and thus, the (neutral) planar form would be expected to exhibit antiaromatic behavior. Indeed, Frost s circle analysis predicts an open-shell species (as with [4]annulene see Figs. 2 and 6). Extensive studies of cyclooctatetraene were performed, and it was established that the neutral state avoids the problem of antiaromaticity by adopting a tub conformation. Both the anion and dianion are known to exist in a planar conformation, the latter being aromatic. This fact is also predicted by simple analysis of the [8]annulene dianion by Frost s circle (see Fig. 6). It is predicted that the planar [8]annulene dianion is a closed-shell system. 

---