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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.

Physical Properties

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

v160+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

Dt = 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° C90100110120130140150160
p (mm.)183567117191294445670
D (grams per c.c.)0.031970.033320.035690.03929...0.0222860.0234480.024913

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

t° C90100110120140150160
Q Cals. per mol14.215.616.616.915.614.914.9

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° C182200230250274300327336
Rel. Density (air = l)5.0784.8514.3023.9913.8403.6743.6563.656
Per cent, dissociation41.748.567.480.087.596.297.397.3

Dissociation pressures of PCl5

t° C90100120140160
Gram-mols. PCl5 per litre0.039450.001590.004460.010960.02368
Pressure in mm.1835117294670

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 Kp = 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.

Chemical Properties

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.

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