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.

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

v_t = 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° C	0	40	100	172.9
v	1.000	1.0348	1.0961	1.1714

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	-20	0	20.8	50.3	75.7	99.8	154	170
D	2.972	2.923	2.871	2.799	2.735	2.676	2.542	2.502
σ	45.8	44.7	43.2	41.3	38.9	36.0	28.4	26.3
σ (VM)2/3	927 0	916.8	894.7	870.0	832.1	781.4	637.9	597.0

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

Parachors of PBr₃

t° C.	24	33	59.5	72.0
D	2.883	2.861	2.795	2.764
σ	45.8	44.1	38.1	37.1
[P]	244.4	244.0	241.2	242.0

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

The dielectric constant at 20° c. is 3.88.

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.