Phosphorus Pentachloride, PCl₅

Phosphorus Pentachloride, PCl₅, is produced by the action of excess of chlorine upon the trichloride until the mass is completely solid, and was thus prepared by Davy. Its composition was established by Dulong's analysis, but the vapour density was found to be lower than the value which corresponds to simple molecules by Avogadro's law. The observed densities were 5.08 at 182° C. and 3.65 over 300° C., the latter density being about half the theoretical, namely, 7.22 (air = 1). This was one of the earliest known examples of abnormal vapour densities, and the abnormality was rightly attributed to a partial or nearly complete dissociation of the compound into PCl₃ and Cl₂. This dissociation is shown by an increasingly greenish-yellow colour in the vapour, originally colourless. In accordance with the law of concentration action the dissociation was diminished by vaporising the compound in an atmosphere of PCl₃ in such a way as to increase the partial pressure of this vapour. The measured density of the PCl₅ was thus brought nearer to the normal value.

The pentachloride is best prepared in a stock bottle provided with entry and exit tubes for the chlorine and a tap-funnel for the trichloride and cooled externally. It is kept filled with chlorine, into which the trichloride is introduced, drop by drop.

Another convenient method is passing chlorine into a solution of the trichloride in carbon disulphide. The pentachloride is then precipitated.

The density of the liquid under the pressure of its own vapour was determined by means of a glass dilatometer, into which was melted a suitable quantity of the compound and which was then evacuated and sealed. The volume of the liquid was read at various temperatures. The weight was obtained by difference, as well as from the weight of silver chloride which was obtained from the contents after hydrolysis. Another method which was found suitable for this hygroscopic substance employed glass floats about 5×1×1mm. which were heated in sealed tubes containing the liquid until they sank. They were calibrated by means of similar observations in a suitable mixture of bromoform and benzene. By the first method the density of the liquid by a slight extrapolation was found to be 1.601 at its boiling-point. The specific volumes were given by the equation

v_160+t = v₁₆₀(1+0.00107t)

The specific volumes were 0.629 and 0.6483 and the corresponding densities 1.590 and 1.554 at 160° and 181° C. respectively. By the second method the density was given as

D_t = 1.624 - 0.00208(t - 150°)

From this the densities are 1.603 and 1.559 at 160° and 181° C. respectively.

The sublimation temperature has been estimated at 160° C., 160° to 165° C., 162.8° C. A thermometer suspended in the vapour of the compound which was subliming freely and condensing on the bulb showed 160° C. An extrapolation of the vapour pressure curve (see below) would give a somewhat higher temperature.

The melting-point (166.8° C.) lay only slightly above the sublimation temperature. The liquid which had been formed in a closed tube under the pressure of its own vapour began to solidify at 162° C.

The coefficient of expansion of the liquid is stated above.

The dissociation pressure (total) at various temperatures, based on measurements with the isoteniscope—

t° C	90	100	110	120	130	140	150	160
p (mm.)	18	35	67	117	191	294	445	670
D (grams per c.c.)	0.03197	0.03332	0.03569	0.03929	...	0.022286	0.023448	0.024913

may be used to calculate the heat of vaporisation with dissociation—

t° C	90	100	110	120	140	150	160
Q Cals. per mol	14.2	15.6	16.6	16.9	15.6	14.9	14.9

The critical temperature determined in the usual manner by observation of the disappearance of the meniscus, was found to be 372° C. The ratio between the boiling-point and the critical temperature on the absolute scale was found to be normal.

The relative density (air = l) diminishes rapidly with increase of temperature, as is shown by the tables above and below. At lower temperatures the density corresponds with single undissociated molecules PCl₅, and at 90° C. there is even a slight degree of association.

Relative densities of PCl₅ vapour

t° C	182	200	230	250	274	300	327	336
Rel. Density (air = l)	5.078	4.851	4.302	3.991	3.840	3.674	3.656	3.656
Per cent, dissociation	41.7	48.5	67.4	80.0	87.5	96.2	97.3	97.3

Dissociation pressures of PCl₅

t° C	90	100	120	140	160
Gram-mols. PCl5 per litre	0.03945	0.00159	0.00446	0.01096	0.02368
Pressure in mm.	18	35	117	294	670

In this table the theoretical numbers of gram-mols. PCl₅ per litre if there were no association or dissociation have been calculated from the formula



Total pressures of the gaseous products in equilibrium with PCl₅ can be represented from the foregoing results by the equation

log p = -6724.22/T – 19.1978 log T + 68.9701

By a comparison of the actual with the theoretical densities the degrees of dissociation, x, the partial pressures, p₁, p₂, p₃, of PCl₅, PCl₃ and Cl₂, respectively, and their concentrations can be determined.

The equilibrium constant K_p = p₁/p₂p₃ is expressed as a function of temperature by means of the equation



By applying the Clapeyron equation the heat of dissociation, Q, of PCl₅ was found to be 21.8 Cals. per mol.

The heat of formation of solid PCl₅ from its elements in their usual physical states is given as 104.99 Cals. Or 109.2 Cals., while the heat of decomposition by water is 123.4 or 118.9 Cals.

The liquid PCl₅ has a very low conductivity, both in the pure state and when dissolved in some solvents, e.g. in benzene or PCl₃, but the solution in nitrobenzene was found to have a definite low conductivity.

The chlorine is displaced from phosphorus pentachloride by fluorine with great evolution of heat and the formation of PF₅. Bromine has no action, but iodine is converted into ICl, which gives an addition compound PCl₅.ICl with more of the PCl₅

On account of its ready dissociation the pentachloride is a most powerful chlorinating agent. Examples of this action on the non- metals are as follows:—

Sulphur gives P<>SCl₃ and also reacts according to

PCl₅ + 2S = S₂Cl₂ + PCl₃

Selenium is converted into the monochloride:—

PCl₅ + 2Se = Se₂Cl₂ + PCl₃

Liquid hydrogen sulphide gives PSCl₃.

Arsenic is converted into the trichloride:—

6PCl₅ + 4As = 4AsCl₃ + 6PCl₃

Antimony reacted in a similar manner. The metals, even the noble metals, are converted into chlorides, but at higher temperatures phosphides may be formed.

With acid anhydrides oxychlorides of the non-metal and of phosphorus are usually formed. Thus NO₂ gives NOCl, POCl₃ and Cl₂. Sulphur dioxide even when dry reacted when heated with PCl₅, giving thionyl chloride:—

PCl₅ + SO₂ = SOCl₂ + POCl₃

Sulphur trioxide, when warmed with PCl₅, is slowly converted into pyrosulphuryl chloride:—

PCl₅ + 2SO₃ = S₂O₅Cl₂ + POCl₃

Selenium dioxide was found to give first a mixture of selenyl and phosphoryl chlorides; the latter compound then chlorinated the selenium completely, the final result being the transference of the whole of the oxygen to the phosphorus and the chlorine to the selenium:—

SeO₂ + PCl₅ = SeOCl₂ + POCl₃ 3SeOCl₂ + 2POCl₃ = P₂O₅ + 3SeCl₄

Phosphorus pentoxide gives phosphoryl chloride, possibly through an intermediate addition compound. In the case of phosphorus trioxide a reducing action was evident, phosphorus trichloride being formed along with the oxychloride. The reaction with arsenic trioxide and probably with arsenic pentoxide was found to produce AsCl₃ and at the same time phosphoryl chloride in the case of the pentoxide. The oxides of boron and silicon were both chlorinated to BCl₃ and SiCl₄, respectively, on heating with PCl₅, preferably in sealed tubes.

Metallic oxides as well as metals are usually converted into chlorides by the pentachloride, while metallic sulphides usually give the chlorides and phosphorus sulphides.

Although PCl₅ is a saturated compound it is capable of forming addition compounds. Among those which have been described are: PCl₅.AsCl₃, PCl₅.AsCl₅, PCl₅.SbCl₅, 2PCl₅.3HgCl₂, PCl₅.FeCl₃, PCl₅.CrCl₃.

Several ammoniates have been described. A white crystalline substance, PCl₅.8NH₃, was precipitated when ammonia was passed into a solution of PCl₅ in CCl₄.

A most important class of reactions in which PCl₅ plays a part is its hydrolysis by water or other compound containing the hydroxyl group with the substitution of chlorine for hydroxyl. Complete hydrolysis with an excess of water gives orthophosphoric and hydrochloric acids:—

PCl₅ + 4H₂O = H₃PO₄ + 5HCl

The heat of hydrolysis is 123.4 Cals., 118.9 Cals. Steam may produce in the first place phosphoryl chloride, thus:—

PCl₅ + H₂O = POCl₃ + 2HCl

The formation of chlorosulphonic acid, SO₂(OH)Cl, as well as of pyrosulphuryl chloride, S₂O₅Cl₂, by the action of PCl₅ on sulphuric acid has given important information as to the constitution of this acid.

Phosphorus pentachloride is one of the most powerful reagents by which the hydroxyl of organic compounds can be replaced by chlorine. Alkyl chlorides, RCl, from alcohols, and acid chlorides, RCOCl, from acids, are often prepared by this method. The pentachloride is thereby converted first into the oxychloride, POCl₃, which may itself be used for the substitution of OH by Cl.