In benzene and hexatriene every carbon atom is sp² hybridized with one p orbital available to overlap with its neighbours. The uninterrupted chain of p orbitals is a consequence of having alternate double and single bonds. When two double bonds are separated by just one single bond, the two double bonds are said to be conjugated. Conjugated double bonds have different properties from isolated double bonds, both physically (they are often longer, as you have just seen) and chemically (see Chapters 22). You have already met several conjugated systems: lycopene at the start of this chapter and β-carotene in Chapter 3, for example. Each of the 11 double bonds in β-carotene is separated from its neighbour by only one single bond. We again have a long chain in which all the p orbitals can overlap to form molecular orbitals.

It is not necessary to have two C=O double bonds in order to have a conjugated system—the C=C and C=O double bonds of propenal (acrolein) are also conjugated. What is important is that the double bonds are separated by one and only one single bond. Here’s a counter-example: arachidonic acid is one of the fabled ‘polyunsaturated’ fatty acids. None of the four double bonds in this structure are conjugated since in between any two double bonds there is an sp³ carbon. This means that there is no p orbital available to overlap with the ones from the double bonds. The saturated carbon atoms ‘insulate’ the double bonds from each other and pre vent conjugation.

If an atom has two double bonds directly attached to it, that is, there are no single bonds separating them, again no conjugation is possible. The simplest compound with such an arrangement is allene. The arrangement of the p orbitals in allene means that no delocaliza tion is possible because the two π bonds are perpendicular to each other.

● Requirements for conjugation

• Conjugation requires double bonds separated by one single bond.

• Double bonds separated by two single bonds or no single bonds are not conjugated.