Phosphorus Pentachloride
Properties
| State | Solid (yellowish-white crystalline solid, fuming in moist air) |
| Color | Yellowish-white |
| Solubility | Reacts with water (hydrolysis to H3PO4 and HCl); soluble in carbon disulfide and carbon tetrachloride |
| Melting Point | 160°C (sublimes under pressure; decomposes at 167°C) |
| Boiling Point | Sublimes at 160°C |
About Phosphorus Pentachloride
Phosphorus pentachloride is the textbook example of a hypervalent main-group compound and the workhorse chlorinating reagent for jobs PCl3 and SOCl2 won't finish. In the gas phase it adopts a clean trigonal bipyramidal geometry — three equatorial P-Cl bonds at 202 pm and two axial bonds at 214 pm — which is the canonical AX5 case in every VSEPR lecture. The bond-length difference is the experimental signature that axial positions feel three pairs of equatorial repulsion versus two for an equatorial position. In the solid state PCl5 abandons that molecular structure entirely and crystallizes as ionic [PCl4]⁺[PCl6]⁻, with a tetrahedral cation and an octahedral anion sitting in a CsCl-type lattice. The same compound, two completely different bonding pictures depending on phase — one of the cleanest experimental demonstrations of how molecular geometry responds to packing constraints. As a reagent, PCl5 converts carboxylic acids to acid chlorides cleanly, alcohols to alkyl chlorides (often with rearrangement), aldehydes and ketones to gem-dichlorides (the only easy reagent for this transformation), and P=O groups to P-Cl. It's prepared by chlorinating PCl3 with excess Cl2 and is so moisture-sensitive that an open bottle visibly fumes within seconds as it hydrolyzes to phosphoric and hydrochloric acids.
Where you'll encounter it
If you've ever made an acid chloride from a carboxylic acid in a synthesis lab and SOCl2 didn't push the reaction to completion, PCl5 is the next reagent off the shelf — it works on hindered substrates like 2,4,6-trimethylbenzoic acid where SOCl2 stalls. Process chemists making chlorpyrifos, glyphosate intermediates, and other organophosphate agrochemicals run continuous PCl3-to-PCl5 chlorination loops where the PCl5 gets consumed in situ for ester chlorination steps. The classic teaching demo is to drop a small chip of PCl5 into a watch glass on the front bench: it immediately starts smoking white HCl mist as ambient humidity hydrolyzes the surface, and the smell of HCl reaches the back row in under a minute. In analytical labs, PCl5 still shows up as the chlorinating agent of choice for converting hydroxyl groups to chlorides on natural-product scaffolds where the substrate can't survive thionyl chloride's acidic byproducts.
Common Uses
- Chlorination of carboxylic acids to acid chlorides where SOCl2 stalls on hindered substrates
- Conversion of ketones and aldehydes to gem-dichlorides (CR2=O to CR2Cl2)
- Synthesis of organophosphate pesticides including chlorpyrifos and parathion intermediates
- Friedel-Crafts catalyst for arene acylation in pharmaceutical fine chemistry
- Classroom demonstration of hypervalent bonding and gas-vs-solid structural duality
Safety Information
GHS H314 (causes severe skin burns and eye damage), H330 (fatal if inhaled), H335 (may cause respiratory irritation), EUH014 (reacts violently with water). Reaction with moisture liberates HCl gas and phosphoric acid mist; even ambient humidity causes visible fuming. ACGIH TLV 0.1 ppm (0.85 mg/m³) TWA; OSHA has no specific PEL but applies the corrosive-substance general standard. Store under nitrogen in a desiccated cabinet; never open in a humid lab. Spills require dry sand or vermiculite cleanup before any aqueous neutralization. Full face shield, butyl gloves, and acid-resistant apron required for any open handling.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.