Alkenes cyclooctatetraene like

Chemically, cyclooctatetraene behaves like an alkene not like benzene. Bromine, for example, does not form a substitution product but an addition product. There is something strange going on here—why is benzene so different from other alkenes and why is cyclooctatetraene so different from benzene The mystery deepens when we look at what happens when we treat cyclooctatetraene with powerful oxidizing or reducing agents. 

Chemically, cyclooctatetraene behaves like an alkene, not like benzene. With bromine, for example, it forms an addition product and not a substitution product. So benzene is not special just because it is cyclic—cyclooctatetraene is cyclic too but does not behave like benzene. 

For example, cyclooctatetraene resembles benzene in that it is a cyclic molecule with alternating double and single bonds. Cyclooctatetraene is tub shaped, however, not planar, so overlap between adjacent n bonds is impossible. Cyclooctatetraene, therefore, is not aromatic, so it undergoes addition reactions like those of other alkenes. 

There is much evidence that cyclic conjugated systems of An electrons show no special stability. Cyclobutadiene dimerises at extraordinarily low temperatures (>35K).28 Cyclooctatetraene is not planar, and behaves like an alkene and not at all like benzene.29 When it is forced to be planar, as in pentalene, it becomes unstable to dimerisation even at 0 °C.30 [12]Annulene and [16]annulene are unstable with respect to electrocyclic reactions, which take place below 0 °C.31 In fact, all these systems appear on the whole to be significantly higher in energy and more reactive than might be expected, and there has been much speculation that they are not only lacking in extra stabilisation, but are actually destabilised. They have been called antiaromatic 32 as distinct from nonaromatic. The problem with this concept is what to make the comparisons with. We can see from the arguments above that we can account for the destabilisation... 

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). 

Cyclooctatetraene, which has eight 71 electrons, seems to fit the category of antiaromatic polyenes since it has 4 ti electrons ( = 2). Nevertheless, cyclooctatetraene is a stable molecule, and reacts like an alkene. For example, it undergoes addition reactions with bromine and is easily hydrogenated. Cyclooctatetraene is not planar. It is not antiaromatic because it exists in a tub conformation, so its 71 orbitals cannot overlap to form a continuous 7t system, which for 871 electrons would be very unstable. Therefore, cyclooctatetraene does not exhibit the general characteristics of either aromatic or antiaromatic compounds. It is nonaromatic. 

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