Warum ist bei einer C=C-Doppelbindung eine pi- und eine Sigmabindung vorzufinden?

You can describe the double bond in one molecule in two ways:

• Either you have sp2-hybridized the carbon, then two hybrid orbitals go into the bonds with the hydrogens and one into the CC-σ bond. The last p-orbital remains unhybridized and forms the CC-π binding.
• Or you make the carbon sp3 and use two of these hybrid orbitals for the CC double bond.

The two are equivalent to zero approximation as regards total energy. However, the σ/π model can easily and correctly explain the chemical reactivity, namely the opening of the weaker π bond while maintaining the stronger σ bond. For the σ- and π-bindings are independent inherent functions to the approximated Hamilton operation, the banana bonds are formed from this by an orthogonal transformation in the space of the occupied orbitals, and one can only do if both bonds are occupied intact and dop-pelt; this is the case only in the undisturbed molecule, but not when something is attached to the π bond, or an electron goes out; Therefore, the banana model is not suitable if you want to make reactions, ionization or electronic suggestions.

Finally, the σ/π model is the more solid.