Pi bonds involve the electrons in the leftover p orbital for each carbon atom. Those p orbitals are the electron clouds or orbitals that are shown going up above and below each carbon atom.
The combination of a sigma and a pi bond between the same two carbon atoms is a double bond. A double bond consists of a sigma bond (using hybrid orbitals) and a pi bond (using p orbitals).
Another way of showing how two carbon atoms can form a double bond is indicated in the bottom (part c) of this diagram. Here we have two carbon atoms with sp2 hybridization, and each of those carbon atoms is bonded to two hydrogen atoms and also to the other carbon atom. Notice that this diagram shows how the carbon atoms and the hydrogen atoms are all in a flat plane. This is drawn in perspective. Notice that there is a sigma bond between the carbon atoms and each of the hydrogen atoms. Notice also a sigma bond between the two carbon atoms. The sigma bonds (or sigma orbitals) are shown as the dark shaded areas in this drawing. Each one of those sigma bonds uses one of the hybrid orbitals. Remember that with sp2 hybridization, there are three hybrid orbitals, and those are the ones used to form the sigma bonds between all the atoms.
Remember also that we have a leftover p orbital for each carbon atom. Those p orbitals are the electron clouds or orbitals that are shown going up above and below each carbon atom. In this particular diagram, the shading between those p orbitals shows that the p orbitals overlap one another and allow the electrons in those p orbitals to be shared. This kind of bond is called a pi bond. The pi bond results when p orbitals overlap one another in this side-to-side fashion.
Let me point out some important structural consequences of a double bond.
Model kits are generally inadequate for showing this sigma-pi nature of a bouble bond. In the model kits available to you, the double bonds are represented using either springs or curved pieces of plastic.
These models might lead you to think that double bonds are formed by some kind of process, in which normal single bonds are bent around to form a curved bond, but that is just not true at all. It is just the simplest way to indicate a double bond using models like this. So keep that in mind when you use models that include double bonds.
The models are not all bad, however. When you make a model of this molecule as part of your lab work, you will note that it does show that all six atoms lie in a flat plane and that you cannot twist or turn or rotate this model like you could the alkanes that you made earlier, which had only single bonds.
E-mail instructor: Eden Francis
Clackamas Community College