In 1865 John Maisch published a short paper "On the Active Principle of Rhus Toxicodendron". For the unsensitized, rhus toxicodendron is the botanical name for poison ivy. Maisch isolated a fraction he considered to be the "active principle" responsible for the misery that is poison ivy and dubbed it toxicodendric acid. Are you itchy yet? (I am and Maisch surely was, he and various visitors to his lab suffered with outbreaks of poison ivy.)
By 1897 Franz Pfaff of Harvard had weighed in. Toxicodendric acid extracted from poison ivy turned out to be acetic acid - yes, vinegar, by another name, CH3COOH. He showed the itch was in the oil.
13 minutes ago in The Phytophactor