In a Diels-Alder reaction, which component acts as the diene?

• A. A conjugated diene
• B. A simple alkene
• C. An alkyne
• D. A dienophile

Answer: (A)

In a Diels-Alder reaction the diene must be a conjugated diene because the reaction is a concerted [4+2] cycloaddition that uses four π electrons from the diene and two π electrons from the dienophile to form the new six-membered ring. A conjugated diene, like 1,3-butadiene, has two adjacent double bonds whose π systems are delocalized, supplying the necessary 4 electrons in a single, coordinated flow during bond formation. A simple alkene only provides two π electrons and cannot supply enough for the [4+2] process, while an alkyne carries two π electrons in a triple bond and does not offer the required two separate, conjugated π bonds to act as the diene. The dienophile is the partner that supplies the two electrons it brings in to complete the cycloaddition, not the diene. Therefore, the component acting as the diene is a conjugated diene.