[4 +2] Cycloaddition
[4 +2] Cycloaddition Definition:
A pericyclic reaction, also called a Diels-Alder reaction, that takes place between two different pi systems, one
of which is associated with four atoms while the other is associated with two atoms.
[4 +2] Cycloaddition Explained:
In a Diels-Alder reaction, there are two different π systems involved. The first involves 4 atoms while the second involves two atoms, as shown in the diagram below:
As you can see, the dienophile attacks the diene (the substituted alkene seeking a diene to react with) in a concerted process (meaning it happens all at the same time), forming a cyclohexene.