Phosphorus pentachloride

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Phosphorus pentachloride
Names
IUPAC name
Phosphorus pentachloride
Other names
Pentachlorophosphorane
Phosphorus(V) chloride
Properties
PCl5
Molar mass 208.22 g/mol
Appearance Colorless solid
Odor Pungent, HCl-like
Density 2.1 g/cm3
Melting point 160.5 °C (320.9 °F; 433.6 K)
Boiling point 166.8 °C (332.2 °F; 439.9 K) (sublimation)
Reacts
Solubility Reacts with alcohols, carboxylic acids
Soluble in benzene, carbon disulfide, carbon tetrachloride, chloroform, dichloromethane, liq HCl
Readily soluble in TiCl4, thionyl chloride
Sparingly soluble in cyanogen chloride, phosphoryl chloride, SnCl4, sulfuryl chloride
Insoluble in VOCl3
Vapor pressure 1.11 kPa (80 °C)
4.58 kPa (100 °C)
Thermochemistry
364.2 J·mol-1·K-1
457.1 kJ/mol
Hazards
Safety data sheet Sigma-Aldrich
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
660 mg/kg (rat, oral)
205 mg/kg (rat)
Related compounds
Related compounds
Phosphorus trichloride
Phosphoryl chloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Phosphorus pentachloride is a chemical compound with the formula PCl5, a phosphorus chloride. It is widely used as a chlorinating reagent.

Properties

Chemical

Phosphorus pentachloride is sensitive to water and will readily hydrolyze in the presence of moisture to give phosphoryl chloride:

PCl5 + H2O → POCl3 + 2 HCl

If excess water is present, the hydrolysis continues until phosphoric acid is obtained:

PCl5 + 4 H2O → H3PO4 + 5 HCl

Like phosphorus trichloride, phosphorus pentachloride is a good chlorinating agent and is capable of chlorinating carboxylic acids to acyl chlorides:

PCl5 + R-COOH → POCl3 + R-COCl + HCl

PCl5 exists in equilibrium with PCl3 and chlorine, at 180 °C the degree of dissociation is ~40%. Because of this equilibrium, samples of PCl5 often contain chlorine, which imparts a greenish coloration. Vacuum can be used to remove the two side products from the pentachloride.

Physical

Phosphorus pentachloride is a fuming white solid, which reacts violently with water and alcohols. It is soluble in many organic solvents such as benzene, chlorinated carbons, dichloroacetic acid, liq. HCl, molten TeCl4, liq. SO2, slightly soluble in arsenic trichloride, liq. CO2 but insoluble in liq. Cl2.

Availability

Phosphorus pentachloride is sold by various chemical suppliers, but due to its hazards it's extremely difficult to acquire. It's best to make it yourself.

Like phosphorus trichloride, PCl5 is listed on Schedule 3 of the Chemical Weapons Convention, which means all transactions may be be monitored.

Preparation

Phosphorus pentachloride can be prepared by chlorinating phosphorus trichloride with elemental chlorine.

PCl3 + Cl2 → PCl5

Sciencemadness user Magpie was able to synthesize small amounts of PCl5, by reacting excess chlorine with white phosphorus dissolved in chloroform, using garage chemist's procedure. See the thread here.

Reacting elemental chlorine with calcium phosphate, silica and carbon at 1300 °C will give phosphorus pentachloride.[1]

Reaction of sulfuryl chloride with elemental phosphorus will give phosphorus pentachloride. If thionly chloride is used instead, phosphoryl chloride will also be produced.[2]

Burning phosphine in a chlorine atmosphere will produce PCl5.[3]

Projects

While very useful in preparing a huge variety of phosphorus compounds and more, the great toxicity of phosphorus pentachloride and its tendency to hydrolyze means you will need a good ventilation system to safely handle the chemical.

  • Chlorinate organic compounds
  • Make phosphoryl chloride
  • Make alkyl and acyl chlorides

Handling

Safety

Phosphorus pentachloride is very corrosive and toxic. In moist air, it hydrolyzes releasing hydrochloric acid fumes, which are very corrosive.

Storage

Phosphorus pentachloride is notoriously hard to store. Sciencemadness user woelen noted that PCl5 tends to attack even teflon-lined caps over time. Ampouling is the only safe method to store this compound for long periods of time.

Disposal

Phosphorus pentachloride should be neutralized with a base, such as calcium hydroxide suspension outside or in a well ventilated area. Always add small amounts of PCl5 to prevent splashing. Even so, there will be plenty of HCl fumes generated by the hydrolysis. Try not to use carbonates as neutralizing agents, as they will fizzle and cause some PCl5 to become airborne.

References

  1. http://www.google.com/patents/US1730521
  2. North, H. B.; Thomson, J. C.; Journal of the American Chemical Society; vol. 40; (1918); p. 774 - 777
  3. Thomson, T.; Philosophical Magazine (1798-1977); vol. 8; (1816); p. 87 - 93

Relevant Sciencemadness threads