Chemical elements
  Phosphorus
    Isotopes
    Energy
    Preparation
    Applications
    Physical Properties
    Chemical Properties
      Alkali Phosphides
      Alkaline Earth Phosphides
      Copper Silver and Gold Phosphides
      Zinc Group Phosphides
      Aluminium Phosphide
      Titanium Group Phosphides
      Tin Phosphides
      Lead Phosphides
      Arsenic Phosphides
      Antimony Phosphides
      Bismuth Phosphides
      Chromium Phosphides
      Molybdenum and Tungsten Phosphides
      Manganese Phosphides
      Iron Phosphides
      Cobalt Phosphides
      Phosphonium Chloride
      Phosphonium Bromide
      Phosphonium Iodide
      Hydrogen Phosphides
      Alkylphosphines
      Phosphorus Trifluoride
      Phosphorus Pentafluoride
      Phosphorus Trifluorodichloride
      Phosphorus Trifluorodibromide
      Fluophosphoric Acid
      Phosphorus Dichloride
      Phosphorus Trichloride
      Phosphorus Pentachloride
      Phosphorus Chlorobromides
      Phosphorus Chloroiodides
      Phosphorus Tribromide
      Phosphorus Pentabromide
      Phosphorus Diiodide
      Phosphorus Triiodide
      Phosphorus Oxytrifluoride
      Phosphorus Oxychloride
      Pyrophosphoryl Chloride
      Metaphosphoryl Chloride
      Phosphoryl Monochloride
      Phosphoryl Dichlorobromide
      Phosphoryl Chlorodibromide
      Phosphoryl Tribromide
      Metaphosphoryl Bromide
      Phosphoryl Oxyiodides
      Phosphorus Thiotrifluoride
      Phosphorus Thiotrichloride
      Phosphorus Thiotribromide
      Mixed Phosphorus Thiotrihalides
      Phosphorus Suboxides
      Phosphorus Trioxide
      Phosphorus Dioxide
      Phosphorus Pentoxide
      Hypophosphorous Acid
      Phosphorous Acid
      Meta- and Pyro-phosphorous Acids
      Hypophosphoric Acid
      Tetraphosphorus Trisulphide
      Diphosphorus Trisulphide
      Tetraphosphorus Heptasulphide
      Phosphorus Pentasulphide
      Phosphorus Oxysulphides
      Phosphorus Thiophosphites
      Phosphorus Thiophosphates
      Phosphorus Selenophosphates
      Phosphorus Sulphoselenides
      Diamidophosphorous Acid
      Phosphorus Triamide
      Monamidophosphoric Acid
      Diamidophosphoric Acid
      Triamidophosphoric Acid
      Dimetaphosphimic Acid ≡P=
      Trimetaphosphimic Acid
      Tetrametaphosphimic Acid
      Penta- and Hexametaphosphimic Acid
      Monamidodiphosphoric Acid
      Diamidodiphosphoric Acid
      Triamidodiphosphoric Acid
      Nitrilotrimetaphosphoric acid
      Monothioamidophosphoric Acids
      Thiophosphoryl Nitride
      Di- Tri-imido- and -amido-thiophosphates
      Imidotrithiophosphoric Acid =
      Phosphorus Chloronitrides
      Triphosphonitrilic Chloride
      Tetraphosphonitrilic Chloride
      Pentaphosphonitrilic Chloride
      Hexaphosphonitrilic Chloride
      Heptaphosphonitrilic Chloride
      Triphosphonitrilic Bromide
      Phosphorus Halonitrides
      Phosphorus Nitride
      Phosphine
      Pyrophosphoric Acid
      Phosphoric acids
    Slow Oxidation
    Phosphatic Fertilisers

Phosphorus Tribromide, PBr3






Phosphorus Tribromide, PBr3, was first prepared by Balard who added bromine drop by drop to phosphorus. The reaction is violent, but may be moderated by the use of red phosphorus, by carrying out the reaction in CS2 solution, and by introducing the bromine as vapour. On account of the formation of volatile phosphorus compounds it is advantageous to have an excess of bromine, which is easily removed by distillation, any phosphorus pentabromide also being decomposed during this operation.

It is generally prepared by dropping bromine from a tap-funnel on to red phosphorus in a flask. The initial reaction occurs with flashes of flame, and external cooling is desirable. Afterwards the bromine is diluted by the PBr3 and the combination proceeds less vigorously. If the phosphorus is in excess a little remains dissolved in the tribromide, which must be fractionally distilled after the addition of a slight excess of bromine. The reaction can also be carried out in the presence of benzene.

Phosphorus tribromide is a colourless liquid which is often slightly turbid in the cold but becomes clear on warming. It fumes in the air and is hydrolysed in a similar manner to the trichloride.


Physical Properties

The density at 0° C. is given as 2.9249, 2.92311 and 2.923. The density at the boiling-point, 172.9° C., is 2.49541. The vapour density, 135.44 (H = l), corresponds to simple molecules PBr3.

The coefficient of expansion has been expressed by the equation

vt = v0(1 + 0.0384117t + 0.06542892t2 + 0.0818893t3)

between 0° c. and the boiling-point. Equations of this type have also been obtained by Pierre. Relative volumes obtained from Thorpe's equation are:

t° C040100172.9
v1.0001.03481.09611.1714


Other values for the boiling-point are 170.2° C. at 750 mm., 172° C. at 752 mm., 176° to 177° C. at 772 mm. The compound solidifies at -41.5° C., -50° C., and melts at -40° C.

The following table gives relations between the surface tension and temperature:—

Surface tensions, densities and molar surface energies of PBr3

t° C-20020.850.375.799.8154170
D2.9722.9232.8712.7992.7352.6762.5422.502
σ45.844.743.241.338.936.028.426.3
σ (VM)2/3927 0916.8894.7870.0832.1781.4637.9597.0


Another series of results was determined with the object of calculating parachors.

Parachors of PBr3

t° C.243359.572.0
D2.8832.8612.7952.764
σ45.844.138.137.1
[P]244.4244.0241.2242.0


The critical temperature is calculated to be 441° C. The refractive index of the liquid for the D line (nD) is 1.6945 at 19.5° C.

The dielectric constant at 20° c. is 3.88.

Chemical Properties

The energy liberated during the combination of phosphorus and bromine to form PBr3 is manifestly less than in the corresponding case of PCl3. This is confirmed by measurements of the heat of combination—

P (solid) + 1½Br2 (liq.) = PBr3 (liq.) + 42.6 Cals.

The superior affinity of chlorine for phosphorus is shown by the fact that this halogen displaces the bromine from PBr3 giving PCl3.

An excess of bromine combines with PBr3 giving the highly dissociated PBr5 (q.v.).

Oxygen has no effect in the cold, but when passed into boiling PBr3 a vigorous and sometimes explosive reaction may take place with the formation of bromine, POBr3 and P2O5.

The hydrolysis is complete and gives phosphorous and hydrobromic acids. The heat of hydrolysis was found to be 64.1 Cals., i.e. nearly the same as that of PCl3.

Hydrogen sulphide was found to react with PBr3 in a somewhat similar manner to water:—

2PBr3 + 3H2S = P2S3 + 6HBr

Ammonia gave first an ammine and then an amide.

Phosphorus tribromide attacks cork, rubber, wood, etc. It also reacts with hydroxylated compounds such as alcohols, and even with ether. It mixes freely with chloroform, benzene, etc., as well as with AsCl3, SnCl4, etc.
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