What do pi bindings look like

The C = C double bond

Experimental investigations of the C = C double bond have shown that the two bonds are energetically not equivalent. One bond is a normal covalent bond, so it is quite strong, similar to a C-C single bond. The other bond, however, is much weaker, the two binding electrons are much more weakly bound here. That is the reason for the chemical behavior of the C = C double bond, which is considered to be "electron-rich" and is "loved" by nucleophiles.

The strong C-C bond is referred to as the sigma bond, the weak C-C bond as the pi bond. The spherical cloud model cannot explain the formation of these two different bonds. According to this model, both bonds should be equally strong, but significantly weaker than a C-C single bond, since the spherical clouds no longer overlap as much in the double bond as in the single bond.

The ethene molecule with "banana bonds"

Here we see the ethene molecule with two covalent bonds that repel each other and therefore look like two bananas, in fact one speaks here of a "banana bond".

With the orbital model and sp3-hybridized carbon atoms cannot explain the C = C double bond any better. If you were to draw a picture here, it would look exactly like the picture above in the spherical cloud model representation, because the sp3-Hybrid orbitals have almost the same shape as the spherical clouds of the spherical cloud model.


In order to explain the existence of two different C-C bonds in the C = C double bond, the concept of hybridization had to be refined even more. In addition to the sp3Hybridization there is also a sp2-Hybridization and an sp hybridization:

The SP2-Hybridization in the energy diagram

At the sp2-Hybridization, the 2s orbital of the carbon atom is "mixed" with only two 2p orbitals, resulting in three energetically equivalent sp2Hybrid orbitals. What remains is an unchanged 2pz-Orbital.

The sigma bond

The C = C double bond: the sigma bond

In this figure we see the ethene molecule with its two sp2-hybridized carbon atoms. The SP2-Hybrid orbitals overlap with the 1s orbitals of the four H atoms. The sigma bond - shown in red in the picture - arises from the overlap of sp2-Hybrid orbitals of the neighboring carbon atoms. The pz-Orbitals - drawn in blue in the picture - protrude vertically from the plane. Each pz-Orbital consists of two orbital lobes, one below the plane and one above the plane.

The pi bond

The C = C double bond: the pi bond

The pi bond is now created by a slight overlap of the upper and lower orbital lobes of the two pz orbitals. This overlap is quite weak, so the pi bond is also very weak and can be easily cleaved, which explains the chemical behavior of the alkenes well.