(Hive Bee)
07-31-03 23:21
No 450943
      On the chlorides of phosphorus
(Rated as: excellent)

Note of caution: The halides of phosphorus are seriously hazardous to one's health and special precautions have to be taken when handling these compounds. 

Material Safety Data Sheet:
Phosphorus trichloride (PCl3) (
Phosphorus pentachloride (PCl5) (
Phosphorus oxychloride (POCl3) (

taken from Mellor's modern inorganic chemistry, new revised edition 1963, p829-833 :

Phosphorus forms two series of compounds with the halogens, represented by the general formulae PX3 and PX5. It is doubtful if the penta-iodide has been obtained, but otherwise all the members of both series are known. The oxyhalides POX3 are also known.

Phosphorous trichloride

This compound can be made by the action of dry chlorine on yellow phosphorus in the following apparatus :

I found no retort pic in Chemdraw so I used above modification

This consists of a retort containing a layer of sand on which rests a quantity of dry yellow phosphorus, the air having been displaced by carbon dioxide. The retort is heated gently. Chlorine, dried by sulphuric acid, is passed in by means of a movable tube. The distance of this tube is so arranged that the phosphorus does not distill (which occurs if it is too near), neither is the pentachloride formed (which happens if it is too far off). When the action has begun, a tongue of flame projects from the chlorine tube, and the retort needs no further heatingsmile, until at the end it is warmed gently to drive the last of the trichloride into the reciever. The product is purified by redistilling it over yellow phosphorus to convert any pentachloride present into trichloride.
Phosphorus trichloride is a mobile liquid with an unpleasant smell. It boils at 73.5, fumes in air and is hydrolysed by water, forming phosphorous acid and hydrochloric acid:

PCl3 + 3H2O __> H3PO3 + 3HCl

It can be frozen to a solid which melts at -111.8.

Phosphorus pentachloride

PCl5 is best prepared by the action of an excess of chlorine on the trichloride. It is convenient to prepare it in the vessel in which it is to be stored.

Excuses for the crappy drawing. crazy

PCl3 + Cl2 __> PCl5

It can also be made by the direct action of chlorine on yellow phosphorus ; or by the action of sulphur chloride on phosphorus trichloride :

3PCl3 + S2Cl2 __> PCl5 + 2PSCl3

Phosphorus pentoxide when pure is an almost colourless solid; as obtained it is usually pale greenish-yellow. It sublimes below 100, but if heated under pressure melts at 148C. It reacts violently with water, forming first the oxychloride, and then orthophosphoric acid:

PCl5 + H2O __> POCl3 + 2HCl
POCl3 + 3H2O __> H3PO4 + 3HCl

Like phosphorus trichloride, it is a valuable reagent for transforming hydroxyl compounds into chlorides, and finds extensive use for this purpose in organic chemistry.
The vapour density of phosphorus pentachloride varies with temperature. The following values were obtained by J.B. Dumas:

Temperature 182 200 250 300
Vapour density 73.3 70 57.6 52.4

The theoretical value for PCl5 is 104.25 (H2 = 1). It is inferred, therefore, that dissociation is taking place into free chlorine and phosphorus trichloride :

PCl5 <---> PCl3 + Cl2

This has been confirmed experimentally by the facts that starch potassium iodide paper give the characteristic blue coloration produced by chlorine when immersed in the vapour of phosphorus pentachloride; and that the addition of chlorine reduces the amount of dissociation as would be expected from the application of the Law of Mass Action to the equilibrium. Thus, with the notation previously employed we have:

        ----------  =   K

Consequently, if the concentration of chlorine or of phosphorus trichloride be raised, by addition of one or other to the system in equilibrium, some combination will take place and the concentration of the pentachloride will thereby increased. That is, dissociation will be reduced.

Phosphoryl Chloride, Phosphorus Oxychloride, POCl3

This compound can be made by very carefully adding water to phosphorus pentachloride until the solid disappears:

PCl5 + H2O __> POCl3 + 2HCl

It is also made by the gradual addition of powdered potassium chlorate to phosphorus trichloride at ordinary temperatures, and distillation of the mixture :

3PCl3 + KClO3 __> 3POCl3 + KCl

It is now manufactured from finely ground phosphate rock which is mixed with activated coke and then treated with chlorine (sometimes mixed with carbon monoxide) at 400.

Ca3(PO4)2 + 6Cl2 + 6C __> 2POCl3 + 3CaCl2 + 6CO,
Ca3(PO4)2 + 6Cl2 + 6CO __> 2POCl3 + 3CaCl2 + 6CO2

Phosphorus oxychloride is a colourless fuming liquid which boils at 107.2 and can be solidified to a colourless crystalline mass melting at -1.25. In aqueous solution it is slowly hydrolysed to phosphoric and hydrochloric acids.

Kluwer's Chemisch-technisch Handboek, 1st print 1954 has the following information:

PCl3: mol wt. 137.35, mp -91, bp 76
Decomposed by water and alcohol. Soluble in benzene, CS2 and ether. Made by "burning" phosphorus in a stream of dry chlorine, a yellow flame is observed.

PCl5: mol wt 208.27, sublimes at 162.
Decomposed by water and alcohol. Soluble in carbon tetrachloride and CS2. Prepared by dissolving 150 g white phosphorus in 600 mL CS2 and bubble absolutely dry chlorine in until the solution is totally saturated with chlorine. PCl5 will crystallise out. The carbon disulfide is distilled off on a water bath.

POCl3: mol wt 153.35, mp 2, bp 105.3
Decomposed by water and alcohol. Prepared by the dropwise addition of PCl3 to absolutely dry potassium chlorate.To avoid explosions one adds some pre-made POCl3 to the KClO3. Also from PCl3 and SO2Cl2, giving thionyl chloride as a side-product.

A Dream Within A Dream (
08-29-03 18:58
No 456359
      "On the chlorides of phosphorus"  Bookmark   

Great post man.


Using these compounds in synthesis is not popularized here, but what a shame.
(Hive Bee)
09-02-03 03:22
No 456789
      well,  Bookmark   

Preparing and handling these compounds is not something for the novice or less-equipped chemist to perform.

I guess for many bees cyanuric chloride holds more potential as a safer to handle, unwatched chlorination agent.

A Dream Within A Dream (