Delocalization and conjugation

Lycopene contains only carbon and hydrogen while most pigments contain many other elements but nearly all contain double bonds. This chapter is about the properties, such as colour, of molecules that have several double bonds and that depend on the joining up or conjugation of the electrons in these double bonds. 

In earlier chapters, we talked about basic carbon skeletons made up of O bonds. In this chapter we shall see how, in some cases, we can also have a large 71 framework spread over many atoms and how this dominates the chemistry of such compounds. We shall see how this % framework is respr, iisible for the otherwise unexpected. stability of certain cyclic polyunsaturated compounds, including benzene and other aromatic compounds. We shall also see how this framework gives rise to the many colours in our world. To understand such molecules properly, we need to start with the simplest of all unsaturated compounds, ethenc. 

The structure of ethene (ethylene) is well known. It has been determined by electron diffraction and is planar all atoms are in the same plane) with the bond lengths and angles shown below. The car- q h ond length 108 pm bon atoms are roughly trigonal and the C-C bond distance is shorter than that of a C-C 0 bond. C=C bond length 133 pm 

Important point. Ettiene is not actually formed by bringing t( ethertwo carbon atoms and four hydrogen atoms individual carbon atoms do not hybridize their atomic orbitals and then combine. We are simply trying to rationalize the shapes of molecular orbitals. Hybridization and LCAO are tools to help us accomplish this. 

The two phases are we can think of the molecular orbitals as being and below the o bond 

C-C ic bond made from overlap of p orbital on. each C atom 

C-C cr bonds made from overlap of sp orbital on each C atom 

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