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This video explores the reaction of butadiene with ethylene to produce cyclohexene.

This is the simplest example of the Diels-Alder reaction, for which the German chemists Otto Diels and Kurt Alder were awarded the Nobel Prize in 1950.

Let us begin with some details of electronic bonding for ethylene.

An ethylene molecule can be represented several ways. Some of the ways use lines to represent the shared electron pairs between atoms. These electronic bonds can also be shown as orbitals which present the shape of the electron cloud forming the bond.

There are two important bonding orbitals in every carbon-carbon double bond, the low energy sigma bond, and a high energy pi bond. Each orbital surface encloses about 90 percent of the electron density for the bond.

Here are the hybrid sp² orbitals of two separated CH₂ fragments. As the two sp² hybrids come together, a sigma bond is formed. It is called a sigma orbital because it has cylindrical symmetry, or s-like shape about the bond axis. We can similarly picture the formation of the carbon-carbon pi bond. Pi bond formation involves the merging of two p-orbitals, each lobe overlapping with another lobe to form two sausage-like links between the atoms. It is called a pi bond to signify its p-like shape when viewed along the bond axis.

Both the sigma and pi carbon-carbon bonds form as the atoms come together. Note that the strong sigma bond forms first and the weaker pi bond forms at shorter distances. The presence of a sigma bond and a pi bond is also represented by the two stick double bond.