Phosphorus Trichloride: Chlorine over Phosphorus

Phosphorus trichloride turns acids into acid chlorides; here it is the first step to acetic anhydride and aspirin. Gattermann (1914): dry chlorine is passed over yellow phosphorus in a retort full of carbon dioxide; the trichloride distils over and is redistilled at 74 °C. 40 g of phosphorus give 125 to 140 g. Phosphorus, chlorine and the product are all poisons: outdoors, never alone. The picture: phosphorus trichloride in a bottle (Wikimedia Commons).