Cyclooctatetraene kekule forms

One can write two Kekule forms for benzene, and these appear to be related to the stability of the molecule. But one can also write two Kekule forms for cyclobutadiene, and for cyclooctatetraene (Fig. 5.2), so are they not going to be similarly stable ... 

Chemists tried for a long time to synthesize cyclobutadiene, and endless failures are recorded in the literatnre. It was eventually synthesized and is unstable in the sense that it cannot be isolated and put in a bottle. It can, however, be trapped in various ways. But it was evident long ago that cyclobutadiene is not a stable molecule such as benzene. Cyclooctatetraene was synthesized by Willstiitter and Waser in 1911, but it was highly reactive, such as a polyene, and certainly not stable such as benzene. So the Kekule forms written for these molecules seem to be misleading. Cyclobutadiene and cyclooctatetraene do not have any particular stability and are not at all like benzene. Why is that ... 

Figure 5.2. Kekule forms for benzene, cyclobutadiene, and cyclooctatetraene.

For the double bonds to be completely conjugated, the annulene must be planar so the p orbitals of the pi bonds can overlap. As long as an annulene is assumed to be planar, we can draw two Kekule-like structures that seem to show a benzene-like resonance. Figure 16-3 shows proposed benzene-like resonance forms for cyclobutadiene and cyclooctatetraene. Although these resonance structures suggest that the [4] and [8]annulenes should be unusually stable (like benzene), experiments have shown that cyclobutadiene and cyclooctatetraene are not unusually stable. These results imply that the simple resonance picture is incorrect. 

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