Cyclooctatetraene, shape

I Cyclooctatetraene has eight it electrons and is not aromatic. The ir electrons are localized into four double bonds rather than delocalized around the ring, and the molecule is tub-shaped rather than planar. 

In accord with the Hiickel rule of 4/t + 2 electrons, both cyciobutadiene and cyclooctatetraene (cot) are nonaromatic. Cyclooctatetraene contains alternating bond lengths and has a tub-shaped conformation ... 

Examine the geometry of cyclooctatetraene dianion. Is it planar If not, describe its shape. Are all the carbon-carbon bond lengths the same If so, are they the same length as those in benzene If not, do they alternate between single and double bonds Do your observations suggest that cyclooctatetraene dianion is aromatic Is this in accord with Hiickel s rule ... 

In contrast to aromatic molecules which have An + 2 n electrons, cyclobutadiene and cyclooctatetraene do not have An + 2 7r electrons and are not aromatic. In fact, diese molecules, which contain An jt electrons (n is an integer), are less stable than die planar model compounds and are termed antiaromatic. Bodi of these molecules adopt shapes that minimize interactions of die n orbitals. 

You will probably be surprised to find cyclooctatetraene (COT for short), unlike benzene, is not planar. Also none of the double bonds are conjugated—there are indeed alternate double and single bonds in the structure but conj ugation is possible only if the p orbitals of the double bonds can overlap here they do not. Since there is no conjugation, there are two C-C bond lengths in cyclooctatetraene—146.2 and 133.4 pm—which are typical for single and double C-C bonds. If possible, make a model of cyclooctatetraene for yourself—you will find the compound naturally adopts the shape below. This shape is often called a tub . 

Of course, this isn t the molecular orbital energy level diagram for real cyclooctatetraene since COT is not planar but tub-shaped. 

Now we can begin to put all the pieces together and make sense of what we know so far. Let us compare the energy level diagrams for benzene and planar cyclooctatetraene. We are not concerned with the actual shapes of the mole-cular orbitals involved, just the energies of them. 

Cyclooctatetraene and derivatives thereof in the ground state have a rigid tub-shaped structure where neighboring double bonds are not planar and cannot participate effectively in Diels-Alder reactions. However, the highly reactive 4-phenyl-3//-l,2,4-triazole-3,5(4//)-dione adds cyclooctatetraene at 20-25 UC to give the [4 + 2] bicyclic adduct with low yield1,2. 

---