Are electrons still delocalized even when there is no ring? To consider this, we’ll look at hexa triene—three double bonds and six carbons, like benzene, but without the ring. There are two isomers of hexatriene, with different chemical and physical properties, because the central double bond can adopt a cis or a trans geometry. The structures of both cis- and trans-hex atriene have been determined by electron diffraction and two important features emerge:

• Both structures are essentially planar.

 • Unlike benzene, the double and single bonds have different lengths, but the central double bond in each case is slightly longer than the end double bonds and the single bonds are slightly shorter than a ‘standard’ single bond. Here’s the most stable structure of trans-hexatriene, with benzene shown for comparison.

The reason for the deviation of the bond lengths from typical values and the preference for planar structures is again due to the molecular orbitals which arise from the combination of the six p orbitals. Just as in benzene, these orbitals can combine to give one molecular orbital stretching over the whole molecule. The p orbitals can overlap and combine only if the molecule is planar.

If the molecule is twisted about one of the single bonds, then some overlap is lost, making it harder to twist about the single bonds in this structure than in a simple alkene. Other planar arrangements are stable, however, and trans-hexatriene can adopt any of the planar con formations shown in the margin.