PCl5

Phosphorus pentachloride, with the chemical formula PCl5, is a phosphorus halide used widely as a chlorinating and dehydrating agent in inorganic and organic synthesis. It is a moisture‑sensitive, white crystalline solid at room temperature that hydrolyzes rapidly in water to give phosphoric acid and hydrogen chloride. In anhydrous media, it serves as a reagent for introducing chlorine into substrates and for preparing other phosphorus chlorides such as POCl3.

Structure and physical properties

In the gas phase, PCl5 adopts a trigonal bipyramidal geometry. The five chlorine atoms occupy two axial and three equatorial positions, with axial P–Cl bonds typically longer and weaker due to greater repulsion in the TBP arrangement. In the liquid and solid states, PCl5 tends to oligomerize to species such as P2Cl10, connected by bridging chloride ligands. The compound is highly reactive with water and oxygen, and as a solid it is typically stored under strictly anhydrous conditions.

Preparation and reactions

PCl5 is prepared by chlorinating phosphorus trichloride:

PCl3 + Cl2 → PCl5

It readily reacts with water:

PCl5 + H2O → POCl3 + 2 HCl

and with excess water to form phosphoric acid and hydrochloric acid:

PCl5 + 4 H2O → H3PO4 + 5 HCl

Uses

As a chlorinating agent, PCl5 converts alcohols to alkyl chlorides under appropriate conditions and converts carboxylic acids to acyl chlorides. It also serves as a dehydrating agent and as a source of chloride for the synthesis of various organophosphorus compounds.

Safety and handling

PCl5 is highly corrosive and releases fumes of HCl upon contact with moisture. It must be stored and handled under strictly anhydrous conditions, with appropriate ventilation and protective equipment.