Phosphorus Halide

any of the compounds that phosphorus forms with halogens. The trihalides (general formula, PX₃; X being a halogen) PF₃, PCl₃, and PBr₃ and the pentahalides (PX₅) PF₅, PCl₅, and PBr₅ are important and have been thoroughly studied. Also known, but less thoroughly investigated, are halides of the general formula PX (PCl, PBr), dihalides of the general formula P₂X₄ (P₂Cl₄, P₂F₄), mixed phosphorus halides of the PFCl₂ and PF₃Cl₂ type, halides containing more than five atoms of halogen (PBr₇, PCl₆I), and oxyhalides (POCl₃, POF₃).

Phosphorus halides are extremely reactive, with reactivity decreasing from the fluorides to the iodides. They can be distilled in a vacuum without decomposition, and they are readily hydrolyzed. Phosphorus halides are able to form compounds of the PCl₃ · 5NH₃ type in nonaqueous media. The most thoroughly studied halides are the trichloride and pentachloride.

Phosphorus trichloride (PCl₃) is a colorless liquid with a melting point of –93.6°C, a boiling point of 76.1°C, and a density at 20°C of 1.575 g/cm³. It is soluble in ether, benzene, chloroform, carbon disulfide, and carbon tetrachloride. The compound is readily hydrolyzed, forming phosphorous and hydrochloric acids. Phosphorus trichloride is obtained through the chlorination of white phosphorus in a solution of PCl₃ (red phosphorus being substituted for white under laboratory conditions). It is used in the synthesis of organophosphorus compounds. PCl₃ is toxic; it causes burns and irritates the eyes and respiratory passages.

Phosphorus pentachloride (PCl₅) occurs as greenish white crystals having a melting point of 167°C (in a sealed tube) and a density of 2.11 g/cm³. It is readily sublimed and is soluble in carbon tetrachloride and carbon disulfide. The compound is hydrolyzed in water to form the oxychloride POCl₃ and hydrochloric acid. Phosphorus pentachloride is obtained through the chlorination of PCl₃. It is used mainly as a chlorinating agent in organic synthesis. The compound is toxic.