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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

Properties

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

vt = 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.

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