Final answer:
Cyclopentadiene undergoes Diels-Alder reactions spontaneously at room temperature because of the strain in its five-membered ring, the high energy of its s-cis conformation, and favorable kinetics allowing it to overcome the activation energy barrier at room temperature.
Step-by-step explanation:
Cyclopentadiene can spontaneously undergo Diels-Alder reactions with itself at room temperature due to its structure containing conjugated dienes and the inherent instability of the molecule. Cyclopentadiene exists in a high-energy state because of the strain in its five-membered ring and the s-cis conformation of the conjugated dienes that is required for the Diels-Alder reaction. When two cyclopentadiene molecules approach each other, they readily form a six-member transition state, which is of lower energy and more stable. The result is a cyclic product known as dicyclopentadiene.
At room temperature, the kinetic energy is enough for dienes to overcome the activation energy barrier, allowing the Diels-Alder reaction to occur spontaneously. Moreover, as this reaction is concerted, which means it takes place in a single step without intermediates, it can proceed quickly and efficiently.