You might assume benzene’s special feature is its ring structure. To see whether this is the case, we’ll look at another cyclic polyene, cyclooctatetraene, with four double bonds in a ring. Given what we have explained about the way that π systems gain stability by allowing overlap between their p orbitals, you may be surprised to find that cyclooctatetraene, unlike benzene, is not planar. There is no conjugation between any of the double bonds—there are indeed alternate double and single bonds in the structure, but conjugation is possible only if the p orbitals of the double bonds can overlap and here they do not. The fact that there is no con jugation is shown by the alternating 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 on the right below. This shape is often called a ‘tub’.

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.

مواضيع ذات صلة

Aromaticity

Conjugation and reactivity: looking forward to Chapter 10

The amide group

Delocalization over three atoms is a common structural feature The carboxylate anion

The allyl system, The allyl anion

The colour of pigments depends on conjugation

UV and visible spectra

The molecular orbitals of butadiene

Conjugation

Multiple double bonds not in a ring

Molecules with more than one C=C double bond Benzene has three strongly interacting double bonds

The structure of ethene (ethylene, CH2=CH2)