Examine the structure of cyclooctatetraene.

•	Cyclooctatetraene (shown below) has four double bonds in a ring, what do you think its 3D structure will be?

 

•	Display the structure of cyclooctatetraene by typing the reference code (refcode) ZZZSAE01 in the Entry box above the structure list on the left hand side of the WebCSD page.

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Inspect the structure using the 3D viewer. You will notice that unlike benzene, cyclooctatetraene is not planar, instead it adopts a “tub” shape. The reason for this lack of planarity is that a regular octagon has internal angles of 135 degrees, while sp2 angles are most stable at 120 degrees. To avoid the strain the molecule therefore adopts a nonplanar geometry.

 

•	Measure each of the carbon-carbon bond lengths in the structure. You should find that there are two carbon-carbon bond lengths: 1.46Å and 1.33Å. These are typical for double and single carbon-carbon bonds.

•	Chemically speaking, cyclooctatetraene behaves like an alkene not like benzene e.g. it does not form a substitution product with bromine, but an addition product.

•	Why is benzene so different from other alkenes and why is cyclooctatetraene different from benzene?