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Concept
Phosphorus trichloride
Summary
Phosphorus trichloride is an inorganic compound with the chemical formula PCl3. A colorless liquid when pure, it is an important industrial chemical, being used for the manufacture of phosphites and other organophosphorus compounds. It is toxic and reacts readily with water to release hydrogen chloride.
Phosphorus trichloride was first prepared in 1808 by the French chemists Joseph Louis Gay-Lussac and Louis Jacques Thénard by heating calomel (Hg2Cl2) with phosphorus. Later during the same year, the English chemist Humphry Davy produced phosphorus trichloride by burning phosphorus in chlorine gas.
World production exceeds one-third of a million tonnes. Phosphorus trichloride is prepared industrially by the reaction of chlorine with white phosphorus, using phosphorus trichloride as the solvent. In this continuous process PCl3 is removed as it is formed in order to avoid the formation of PCl5.
P4 + 6 Cl2 → 4 PCl3
It has a trigonal pyramidal shape. Its 31P NMR spectrum exhibits a singlet around +220 ppm with reference to a phosphoric acid standard.
The phosphorus in PCl3 is often considered to have the +3 oxidation state and the chlorine atoms are considered to be in the −1 oxidation state. Most of its reactivity is consistent with this description.
PCl3 is a precursor to other phosphorus compounds, undergoing oxidation to phosphorus pentachloride (PCl5), thiophosphoryl chloride (PSCl3), or phosphorus oxychloride (POCl3).
PCl3 reacts vigorously with water to form phosphorous acid (H3PO3) and hydrochloric acid:
PCl3 + 3 H2O → H3PO3 + 3 HCl
Phosphorus trichloride is the precursor to organophosphorus compounds. It reacts with phenol to give triphenyl phosphite:
Alcohols such as ethanol react similarly in the presence of a base such as a tertiary amine:
With one equivalent of alcohol and in the absence of base, the first product is alkoxyphosphorodichloridite:
In the absence of base, however, with excess alcohol, phosphorus trichloride converts to diethylphosphite:
PCl3 + 3 EtOH → (EtO)2P(O)H + 2 HCl + EtCl
Secondary amines (R2NH) form aminophosphines.
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