Final answer:
If the diene in a Diels-Alder reaction is in the s-trans conformation, it would not undergo the reaction because the s-trans conformation is non-planar and prevents effective p orbital overlap.
Step-by-step explanation:
If the diene reacting in a Diels-Alder reaction is in the s-trans conformation, it would not undergo the reaction. This is because the s-trans conformation results in the diene being non-planar, which prevents the necessary overlap of the p orbitals for the reaction to occur.
The Diels-Alder reaction requires the diene to be in the s-cis conformation, where the substituents on the diene are on the same side of the double bond. In this conformation, the diene is planar and the p orbitals can overlap effectively, allowing the reaction to take place.