Answer
Cyclopentadiene was the primary focus of Alder's research. This compound is a highly reactive hydrocarbon with a unique five-membered ring structure. Alder's work centered on understanding the chemical reactions of cyclopentadiene, particularly its ability to undergo Diels-Alder reactions with various dienophiles. These reactions led to the formation of complex cyclic compounds, which were of great interest to Alder and other chemists.