The general Diels-Alder reaction combines a diene (here, cis-butadiene) and a dieneophile (here, ethylene) to give a fused ring system (here, cyclohexene). Such reactions serve as the prototype of "Woodward-Hoffmann-allowed" 4+2 cycloadditions. Consistent with the high calculated activation energy (>20 kcal/mol), the rate for the reaction of unsubstituted ethylene + butadiene is immeasurably slow under standard-state conditions. However, this reaction serves as a prototype for many derivative reactions that are observed in the laboratory.