ABSTRACT
In order to describe a carbon-carbon double bond, we need to return to the electronic conƒguration of carbon:
As before, we formally promote one electron to obtain
This time, we set aside the 2pz orbital, which we will use to make a π-bond, and take the 2s, 2px, and 2py orbitals to make three 2p2 hybrid orbitals. These sp2 hybrids point toward the corners of an equilateral triangle (by VSEPR or some serious mathematics), with angles of 120o between them. The calculated orbitals are shown in Figure 3.1. We can now use these orbitals with the 1s orbitals from hydrogen to make the σ-bonds of ethene (3.2). The σ-bonds to hydrogen are each made from a hydrogen 1s and the sp2 orbital. The carbon-carbon σ-bond is formed from two sp2 orbitals, coming together effectively nose to nose. Since we used s, px, and py to make these hybrid orbitals, the σ-framework must be planar in the xy plane (Figure 3.2).
