Final answer:
The Diels-Alder reaction is bimolecular because it involves two reacting species - the diene and the dienophile - that combine in a single reaction step.
Step-by-step explanation:
Since the Diels-Alder reaction occurs in one step, it is said to be bimolecular. This is because the Diels-Alder reaction involves two reacting species, the diene and the dienophile, that come together in a single reaction step to form a new compound. The molecularity of a reaction refers to the number of molecules participating in an elementary reaction step. Since both the diene and dienophile are involved, it is not a unimolecular reaction which involves a single reactant species, nor is it a termolecular reaction that would involve three reactant species.