Final answer:
The Diels-Alder reaction is a concerted reaction that takes place in a single step, forming a six-membered ring from a conjugated diene and an alkene.
Step-by-step explanation:
The Diels-Alder reaction is considered to be a concerted reaction because it occurs in one step. This type of reaction involves a conjugated diene, such as butadiene, reacting with an alkene, like ethene, to form a cyclic six-membered ring. The reaction can be either intermolecular, involving two separate molecules, or intramolecular, within a single molecule. An example of the intramolecular Diels-Alder reaction is seen in the biosynthesis of the antibiotic lovastatin.
A hallmark of the Diels-Alder reaction is that it takes place through a single transition state with no intermediates. Unlike reactions that occur in multiple steps, where different mechanisms such as unimolecular or bimolecular reactions participate, the Diels-Alder reaction does not involve a series of elementary reactions. In other words, the entire process of forming the cyclohexene ring happens in a simultaneous fashion without any discrete steps.