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 on the left. The carbon atoms are roughly trigonal and the C=C bond distance is shorter than that of a typical C–C single bond. The electronic structure of ethene, you will recall from Chapter 4, can be considered in terms of two sp² hybridized C atoms with a σ bond between them and four σ bonds linking them each to two H atoms. The π bond is formed by overlap of a p orbital on each carbon atom.

Ethene is chemically more interesting than ethane because of the π system. As you saw in Chapter 5, alkenes can be nucleophiles because the electrons in the π bond are available for donation to an electrophile. But remember that when we combine two atomic orbitals, we get two molecular orbitals, from combining the p orbitals either in phase or out of phase. The in-phase combination accounts for the bonding molecular orbital (π), whilst the out-of-phase combination accounts for the antibonding molecular orbital (π*). The shapes of the orbitals as they were introduced in Chapter 4 are shown below, but in this chapter, we will also represent them in the form shown in the brown boxes—as the con stituent p orbitals.