Phosphorus Oxychloride, POCl₃

Phosphorus oxytrichloride or phosphoryl chloride, POCl₃, was early formed during investigations into the action of the pentachloride on substances containing the hydroxyl group. It was also prepared by the oxidation of the trichloride by air, by oxygen, and by many oxidising agents, e.g. nitrogen trioxide. It is also produced by the action of water in small proportion or acetic acid on PCl₃Br₂ or by the action of chlorsulphonic acid on red phosphorus.

Two convenient methods of preparation are as follows:—

1. A current of chlorine is passed through phosphorus trichloride and water is added at the same time, drop by drop. The liquid is kept at its boiling-point, and is distilled at the end of the operation, which is signalised by the appearance of the pentachloride.
2. 500 grams of PCl₃ (free from phosphorus) are placed in a tubulated retort having a capacity of 950 c.c. or more. 160 grams of potassium chlorate are added through the tubulure in lots of about 4 grams. The POCl₃ is then distilled:—
   
   3PCl₃ + KClO₃ = 3POCl₃ + KCl
   
   The KClO₃ should be dry and should previously be covered with some POCl₃ before adding to the PCl₃. Dried oxalic acid may be used as the oxidising agent, the weight used being half that of the PCl₃.

Phosphoryl chloride may be produced on a large scale by passing a mixture of chlorine and carbon monoxide over an intimate mixture of carbon and calcium phosphate (such as charred bone) at 330° to 340° C.:—

Ca₃(PO₄)₂ + 2Cl₂ + 2CO = Ca(PO₃)₂ + 2CO₂ + 2CaCl₂
Ca(PO₃)₂ + 4Cl₂ + 4CO = 2POCl₃ + 4CO₂ + CaCl₂

By a similar reaction ferric phosphate is decomposed by carbonyl chloride at 300° to 400° C.

Phosphoryl chloride is a colourless fuming liquid which resembles the trichloride in appearance but boils at a higher temperature, 107° C., and can easily be frozen to a solid which melts slightly above 0° C.

Analysis combined with determinations of vapour density led to the formula POCl₃. According to the results of many investigators the density of the liquid at ordinary temperatures is slightly below 1.7. The exact experiments of Thorpe and Tutton gave the following results for the density at 0° C., 1.71165 to 1.71185 (1st series) and 1.71185 to 1.71190 (2nd series), and at the boiling-point, 107.23° C., 1.50967. The specific volumes and thermal expansions between these temperatures are summarised by the formula

v_t = v₀(1 + 0.0010643092 + 0.0₅112666t² + 0.0₈5299t³)

The liquid boils at 107.22° to 107.33° C. at 760 mm. and at 104.5° to 105.5° C. at 783 mm. It freezes at about -10° C. and the melting-point of the solid was +2° C., +1.78° C. The critical temperature is 329° C.

The surface tension, as determined by the capillary tube method, was given as 31.91 dynes/cm. at 18° C. and 28.37 at 46.1° C. Hence the change of molar surface energy with the temperature is normal. This physical constant, together with some others, was determined in order to obtain the parachor [P]. The sample of POCl₃ had a boiling- point of 108.7° C. at 769 mm.

Parachor of phosphorus oxychloride

t° C	15	49	65
D	1.690	1.626	1.596
σ	32.77	28.36	26.57
[P]	217.1	217.7	218.1

The dielectric constant of the oxychloride is 13.9 at 22° C. The heat of formation is nearly twice that of the trichloride and also greater than that of the pentachloride according to the equation

P (solid) + ½O₂ (gas) + 1½Cl₂ (gas) = POCl₃ (liquid) + 146.0 Cals.,

The heat of formation from the trichloride and oxygen is also considerable, thus

PCl₃ (liquid) + ½O₂ (gas) = POCl₃ (liquid) + 70.66 Cals.

The heat of hydrolysis with water is 74.7 Cals. If much water is used the products are orthophosphoric and hydrochloric acids, but when the compound deliquesces in moist air pyrophosphoryl and metaphosphoryl chlorides (q.v.) are formed as intermediate products. It is also hydrolysed by hydroxylated organic compounds and is much used for preparing chlorinated derivatives, e.g. C₂H₅Cl and CH₃CO,Cl from C₂H₅OH and CH₃COONa respectively. By this means also alkyl chlorophosphates and alkyl phosphates can be prepared, e.g.:—

2C₂H₅OH + POCl₃ = POCl(OC₂H₅)₂ + 2HCl
POCl(OC₂H₅)₂ = C₂H₅Cl + PO₂(OC₂H₅)

The alcohol must be added drop by drop to the phosphoryl chloride cooled in an ice-salt mixture. Sodium ethoxide yields triethyl phosphate:—

3C₂H₅ONa +POCl₃ = PO(OC₂H₅)₃ + 3NaCl

The oxychloride is also hydrolysed by orthophosphoric acid, which is dehydrated to the meta-acid, and by phosphorous acid, which is dehydrated and oxidised to metaphosphoric acid, thus:—

2H₃PO₄ + POCl₃ = 3HPO₃ + 3HCl
2H₃PO₃ + 3POCl₃ = 3HPO₃ + 2PCl₃ + 3HCl

When the oxychloride is treated with potassium chlorate the chlorine of the former is displaced by oxygen, according to the equation

2POCl₃ + KClO₃ = P₂O₅ + 3Cl₂ + KCl

A similar displacement can be effected by sulphur trioxide, thus

2POCl₃ + 6SO₃ = P₂O₅ + 3S₂O₅Cl₂

Hydrogen sulphide gives a phosphorous oxysulphide, and, when the reaction is carried out at 100° C., an oxychlorosulphide to which the formula P₂O₂SCl₄ was assigned, and which could be distilled under reduced pressure.

Phosphorus oxychloride chlorinates the oxides of several non-metals. The chlorides produced may form addition compounds with excess of the oxychloride: thus SeOCl₂ gave SeCl₄ and P₂O₅; TeO₂ gave TeCl₄.POCl₃; B₂O₃ gave BCl₃.POCl₃ as well as BPO₄. Other addition compounds are as follows SbCl₅.POCl₃; TiCl₄.2POCl₃; TiCl₄.POCl₃; AlCl₃.POCl₃; SnCl₄.POCl₃.

Phosphorus oxychloride is a good solvent for various acid chlorides, bromides and iodides such as those of arsenic and antimony, for VOCl₃, S₂Cl₂ and halides of transitional elements such as FeCl₃ and PtCl₄. For this reason, and on account of its relatively high freezing- point, POCl₃ has been much used for the determination of molecular weights by the cryoscopic method.

Combination with dry ammonia in carbon tetrachloride gave a solid product which was said to be the hexammoniate, POCl₃.6NH₃, while gaseous ammonia gave a white solid from which amides and aminochlorides were isolated.

Metals with widely different electro-affinities ranging from the alkalies to silver and mercury were not found to be affected much in the cold, but when heated they gave oxides, chlorides and phosphides in various cases.

Addition compounds such as CaO.2POCl₃ and MgO.3POCl₃ were obtained by heating the constituents together.