The s-cis and s-trans conformations of 1,3-butadiene are critical to its reactivity, particularly in Diels-Alder reactions. The s-cis conformation allows the π electrons in the conjugated diene to align properly with the π electrons of the dienophile, facilitating the formation of the cyclic adduct. In contrast, the s-trans conformation is not reactive in the Diels-Alder reaction because the π systems cannot overlap effectively due to the extended configuration, which places the reactive π bonds too far apart. As a result, only the s-cis conformation can participate in the Diels-Alder reaction, illustrating the importance of molecular geometry in chemical reactivity.