Phosphorus Thiotrichloride, PSCl₃

Phosphorus Thiotrichloride or thiophosphoryl chloride, PSCl₃, has been prepared by many reactions:—

1. The first method, which led to the discovery of this compound, was by the action of H₂S on PCl₅, with elimination of HCl, thus;—
   
   H₂S + PCl₅ = PSCl₃ + 2HCl
   
   The H₂S may be supplied in the liquid form.
2. Some sulphides of the non-metals yield their sulphur to PCl₅ in exchange for chlorine:—
   
   CS₂ + 2PCl₅ = 2PSCl₃ + CCl₄
   P₂S₅ + 3PCl₅ = 5PSCl₃ (in a sealed-tube at 120°)
   
   Compare also:
   
   Sb₂S₃ + 3PCl₅ = 2SbCl₃ + 3PSCl₃
3. Sulphur combines directly with PCl₃ in a sealed tube at 130° C.:—
   
   PCl₃ + S = PSCl₃
4. Sulphur monochloride combines when heated with phosphorus, thus:—
   
   2P + 3S₂Cl₂ = 2PSCl₃ + 4S
5. Sulphur monochloride reacts with phosphorus trichloride in the presence of iodine, thus:—
   
   3PCl₃ + S₂Cl₂ = 2PSCl₃ + PCl₅
6. Thionyl chloride when heated with tetraphosphorus decasulphide in a closed tube at 100° to 150° C. reacts according to the equation
   
   P₄S₁₀ + 6SOCl₂ = 4PSCl₃ + 3SO₂ + 9S

Thiophosphoryl chloride is a transparent colourless liquid which fumes in the air and smells of hydrogen sulphide. The formula has been established by analysis and vapour density determination. The density of the liquid is 1.66820 at 0° C. and 1.45599 at the normal boiling-point, 125.12° C. The coefficient of expansion between these temperatures is given by

v_t = v₀(1 + 0.0₃99011t + 0.0₆90302t² + 0.0₈3825t³)

The melting-point is -35° C.

Thiophosphoryl chloride decomposes when passed through a red-hot tube, giving PCl₃, S and S₂Cl₂. An excess of chlorine combines with both the other elements, according to the equation

2PSCl₃ + 3Cl₂ = 2PCl₅ + S₂Cl₂

The compound is reduced slowly by hydrogen iodide, giving PI₃, H₂S, HCl and sulphides of phosphorus. Like other halides of phosphorus it combines with dry ammonia, giving a white solid, which may contain from 30 to 60 per cent, of NH₃. This product is said to contain thiophosphoryl diaminochloride, P(NH₂)₂ClS, or thiophosphoryl triamine, P(NH₂)₃S.

Thiophosphoryl chloride dissolves sulphur and phosphorus freely when hot, but only sparingly when cold. Since the liquid is immiscible with water the hydrolysis proceeds only on the surface at first, as is usual with phosphorus halides. In this case the products are phosphoric acid, hydrogen chloride and sulphide and a little sulphur. It reacts with ethyl alcohol, giving ethyl chloride and ethyl thiophosphate:

3C₂H₅OH + PSCl₃ = (C₂H₅)H₂PSO₃ + 2C₂H₅Cl + HCl

Thiophosphates can also be made from the thiochloride with sodium ethoxide and aqueous alkalies.