Phosphorus Trichloride, PCl₃

The action of chlorine on phosphorus was investigated by Gay-Lussac and Thenard, and also by Davy. It was shown that the phosphorus burns with a pale flame and that both a liquid and a solid compound are produced. The preparation of the pure liquid alone (namely, the trichloride) is best carried out in a retort (filled with dry carbon dioxide), the bottom being covered by a layer of sand. The dry and clean yellow phosphorus is introduced and melted in a current of dry carbon dioxide. The retort is kept in warm water and a current of dry chlorine is introduced through the inlet tube, which should be adjustable to the distance above the phosphorus which gives a brisk but not too violent reaction. The distillate may be purified by redistilling with a little yellow phosphorus.

Phosphorus may also be combined to form the trichloride by passing its vapour over mercurous or mercuric chloride or cupric chloride, or by passing the vapour of sulphur monochloride over phosphorus, or by heating red phosphorus with sulphuryl chloride. It is possible to prepare it directly from calcium phosphate by heating this with silica and charcoal and passing over it the vapour of sulphur monochloride:—

4S₂Cl₂ + Ca(PO₃)₂ = 2PCl₃ + CaCl₂ + 3SO₂ + 5S

Reactions of some theoretical interest which give the trichloride are:—

1. Heating phosphorus in a sealed tube with HCl:—
   
   2P + 3HCl = PCl₃ + PH₃
2. Reduction of the pentachloride by hydrogen and some metals.
3. Reduction of the oxychloride with charcoal at a red heat:—
   
   POCl₃ + C = PCl₃ + CO₂

Phosphorus trichloride is a colourless liquid which boils at 76° C. and freezes at about -100° C. The vapour density and the analysis correspond to the molecule PCl₃. The liquid fumes in moist air with decomposition. It is immiscible with water, but is completely hydrolysed by it forming hydrochloric and phosphorous acids.

The vapour density has been determined as 4.7464 and 4.75 (air = l). The thermal expansion is 0.00489 from 100° to 125° C. and 0.00417 from 125° to 180° C. The specific heat is 0.1346 to 0.1347 between 111° and 246° C. The refractive index for the D line is given as 1.001730.

The density has been determined by several investigators with the following results in grams per c.c. at 0° C.:—1.6119 (Buff), 1.6162 (Pierre), 1.61275 (Thorpe). The densities can be calculated as the quotients of the density at 0° C. divided by the relative specific volume for other temperatures, i.e. as D_t = D₀/v_t in which v_t, the coefficient of expansion, is given by the equations of Thorpe (below) or that of Pierre. The mean coefficient of expansion between 0° and 75° C. is 0.0013436, and the specific volumes from 0° C. to the boiling-point are given by the equation

v_t = v₀(1 + 0.0011393t + 0.0₅166807t² + 0.0₈4012t³)

Hence the volume at the boiling-point 75.95° C. is 1.09827 times that at 0° C., and the molar volume at the boiling-point is 93.34.

Another series of results has been continued to a lower temperature, so as to include the density of the solid:—

t° C	-95	-95	-80	-50	-30	0.0	+20
D	1.9036 Solid	1.7876 Liquid	1.7601	1.7046	1.6671	1.6128	1.5778

An estimate of the limiting density at -273° C. is 2.11927, and this has been estimated to decrease with rise of temperature according to the equation

D_t = 2.11927 - 0.00189994T + 0.0₆1183T²

The compressibility of the liquid at 10.1° C. between 1 and 500 atmospheres was found to be 0.0₄72. At 20° C. the relative volume of the liquid was reduced from 1.0234 to 0.9862 by a pressure of 500 kilos, per sq. cm. and further to 0.7763 by 12,000 kilos.

The vapour pressure measurements of Regnault were expressed by the equation

logp = 4-7479108 -3-1684558a<9 (between -20° and +50° C.) in which log a = 1-9968895; or by 5

logp = l.2112[5.6885 - 1000(θ + 228)^-1]

The dielectric constant6 at 22° C. is 4.7.

The boiling-point at normal pressure has been given as 74° to 78° C. by different investigators. It is very close to 76° C. The carefully determined values of Thorpe are: 75.95° C. at 760 mm. And 76.25° C. at 768 mm. More recent values are 75.5° C. at 763 mm. and 75° C. at 749 mm.

The latent heat of vaporisation is given as 9.0 Cals. per mol at 0° C. and 6.9 to 7.1 at the boiling-point. The constant Q_LV/T_b = 20 is therefore normal.

The critical temperature has been given as 285.5° C. and as 290.5° C.

The melting-point is low, from -111.5° to -90.0° C.

The surface tension, as determined by Ramsay and Shields, was 28.71 dynes/cm. at 16.4° C., and 24.91 dynes/cm. at 46.2° C. Molar surface energies corresponding to these are 562.3 and 499.8 ergs respectively, the decrease in molar surface energy per degree being 2.097, which is about the normal value. More recent determinations over a larger range are:—

t° C	-70	-20.5	0	+20.8	35.2	75.1
D	1.744	1.653	1.613	1.574	1.547	1.475
σ	37.4	31.6	29.3	27.3	25.8	21.9

The refractive index of the liquid n has been determined at several wavelengths λ (in microns = 10^-3 mm.).

λ	0.263	0.298	0.394	0.589	0.768
n	1.664	1.59983	1.54274	1.51215	1.50340

These results may be compared with the following:—

λ	0.486	0.589	0.759μ
n	1.525	1.516	1.506

The molar depression of the freezing-point for the trichloride in benzene is 0.636° and the molar elevation of the boiling-point in the same solvent is about 4.5°.

Phosphorus trichloride mixes with organic liquids such as benzene and nitrobenzene, as also with the oxychloride, and with sulphur without reaction at ordinary temperatures.

In many respects the trichloride behaves as an unsaturated compound. Although it does not burn in the air it seems to absorb oxygen to some extent and also ozone, giving the oxychloride.

It is oxidised by sulphur trioxide, by concentrated sulphuric acid (to HPO₃), by SOCl₂, SeOCl₂ and KClO₃ as follows:—

PCl₃ + SO₃ = POCl₃ + SO₂
PCl₃ + 2H₂SO₄ = HPO₃ + H.SO₃Cl + SO₂ + 2HCl
3PCl₃ + SOCl₂ = POCl₃ + PCl₅ + PSCl₃
3PCl₃ + 3SeOCl₂ = 3POCl₃ + SeCl₄ + Se₂Cl₂
3PCl₃ + KClO₃ = 3POCl₃ + KCl

The halogens convert it into penta-compounds. Thus fluorine gives PF₅ and chlorine PCl₅. The action of bromine varies with the conditions; substitution may occur, giving, e.g. PCl₂Br₃, as well as PCl₃Br₂ by addition. Iodine may form a chloroiodide. Bromine and iodine together act vigorously according to the equation:—

PCl₃ + 5Br + I = PBr₅ + ICl₃

Phosphorus trichloride is chlorinated by selenium tetrachloride:— 6PCl₃ +7SeCl₄ = 3(PCl₅)₂.SeCl₄ + 2Se₂Cl₂

by antimony pentachloride:—

2SbCl₅ + PCl₃ = PCl₅.SbCl₅ + SbCl₃

and by sulphur monochloride:—

3PCl₃ + S₂Cl₂ = PCl₅ + 2PSCl₃

It chlorinates arsenic (in the presence of a little AsCl₃) at 200° to 300° C., also antimony, phosphine and arsine, giving phosphorus in each case.

The reaction with cold water normally gives a solution of phosphorous and hydrochloric acids:—

PCl₃ + 3H₂O = H₃PO₃ + 3HCl

Intermediate stages in the reaction have been noted, such as the production of phosphoryl monochloride, POCl, with small amounts of water. The solution produced at first has stronger reducing power than the final solution, which was attributed to a first production of P(OH)₃ with subsequent change to the tautomeric OPH(OH)₂. In concentrated or hot solution subsequent decomposition of the phosphorous acid may take place with the production of phosphoric acid and red phosphorus, which change has been represented by the equation

4H₃PO₃ + PCl₃ = 3H₃PO₄ + 3HCl + 2P

The velocity constant of the decomposition, since this takes place at a surface of separation of two immiscible liquids, is that of a unimolecular reaction, i.e. log (a/(a-x)) = kt×aS, in which a is the quantity of liquid and S is the reacting surface.

Phosphorus trichloride is used in numerous reactions with organic compounds containing hydroxyl to replace this radical by chlorine; by this means, for example, the chlorides of aliphatic alcohols, etc. may be obtained. The other product is phosphorous acid.

Heats of hydrolysis and solution in much water are given as 65.14 Cals., 62.3 Cals., 63.3 Cals. Using these and other data the heat of formation of the liquid trichloride from solid phosphorus and gaseous chlorine is 73.3 Cals., 76.6 Cals., 75.8 Cals.

By the action of ammonia on phosphorus trichloride in carbon tetrachloride ammoniates such as PCl₃.6NH₃ and PCl₃.8NH₃ have been obtained. On heating, these ammines are decomposed with the formation of a phosphamide and ammonium chloride.

Phosphorus trichloride reacts with liquid hydrogen sulphide to produce phosphorus trisulphide at ordinary temperatures.