Fulvene is one of several hydrocarbons with the same formula as benzene, C₆H₆. The fulvenes are the class of molecules based on this simple hydrocarbon skeleton; the parent chemical, fulvene itself, is rarely encountered. Thiele is credited with discovering the scope of the reaction between cyclopentadiene and aldehydes and ketones that yields the brightly coloured fulvene derivatives. Most fulvenes are still prepared by this route, starting from cyclopentadiene or its sodium cyclopentadienyl anionic form.