Phosphorus Halide

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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.