The unique character of conjugated dienes manifests itself dramatically in the Diels-Alder Cycloaddition Reaction. A cycloaddition reaction is the concerted bonding together of two independent pi-elec...The unique character of conjugated dienes manifests itself dramatically in the Diels-Alder Cycloaddition Reaction. A cycloaddition reaction is the concerted bonding together of two independent pi-electron systems to form a new ring of atoms. When this occurs, two pi-bonds are converted to two sigma-bonds, the simplest example being the hypothetical combination of two ethene molecules to give cyclobutane. This does not occur under normal conditions, though.
