Nitrosyl Chloride, NOCl

1. Nitrosyl chloride may be prepared in many ways. It was first prepared by Gay-Lussac by the direct union of nitric oxide and chlorine:
   
   2NO + Cl₂ = 2NOCl.
2. The most convenient method for producing pure nitrosyl chloride is by the decomposition of aqua-regia. A mixture of 1 volume of concentrated nitric acid (sp. gr. 1.42) and 4 volumes of hydrochloric acid (sp. gr. 1.16) is gently warmed, and the nitrosyl chloride and chlorine dried by means of calcium chloride. The dried mixture of gases is then led into concentrated sulphuric acid, which forms nitrosyl-sulphuric acid with the nitrosyl chloride, while the chlorine and some hydrogen chloride pass on:
   
   NOCl + SO₂(OH)₂ = + HCl
   
   The nitrosyl-sulphuric acid is decomposed by dropping the saturated sulphuric-acid solution on to dry sodium chloride:
   
   + NaCl = + NOCl.
3. Dry hydrogen chloride passed into liquid nitrogen trioxide evolves nitrosyl chloride, which is not quite pure:
   
   2HCl + N₂O₃ = 2NOCl + H₂O.
   
   The following methods have been used for the preparation of nitrosyl chloride, but none of them give as pure a product as that given under (2): -
4. Distillation of potassium nitrite with phosphorus pentachloride:
   
   KNO₂+ PCl₅ = NOCl + KCl + POCl₃.
5. Nitric oxide decomposes ferric chloride at a high temperature:
   
   FeCl₃ + NO = FeCl₂ + NOCl.
6. Nitrosamines react with hydrogen chloride:
   
   R₂N.NO + HCl = R₂NH + NOCl.

Nitrosyl chloride is a yellow gas at ordinary temperatures, with a suffocating odour. It is readily liquefied by passing through a tube immersed in a freezing mixture, and a red liquid is obtained which boils at –5.5° C. at 760 mm. The chloride is obtained as a yellow solid by further cooling. The melting-point is variously given as -65°, -60° to -61° C.

The variation in density of liquid nitrosyl chloride with temperature is given in the following table: -

Temperature, ° C.	-12	-15	-18	-55
Density	1.4165	1.4250	1.4330	1.550

The critical temperature is 163° to 164° C.

Nitrosyl chloride does not begin to dissociate until the relatively high temperature of 700° C. is reached. The dissociation into nitric oxide and chlorine is attended with the absorption of -14.4 Cals.

The absorption spectrum shows three bands in the red and three feeble bands in the extreme green.

Nitrosyl chloride is immediately decomposed by water with the formation of hydrochloric and nitric acids. Most metals react with the gas (gold and platinum are unaffected) to give the metallic chloride and nitric oxide:

Hg + NOCl = HgCl + NO.

Metallic oxides and hydroxides form corresponding nitrites and chlorides:

2KOH + NOCl = KNO₂ + KCl + H₂O.

Many metallic chlorides form additive compounds with nitrosyl chloride, e.g. CuCl.NOCl, BiCl₃.NOCl, ZnCl₂.NOCl, FeCl₃.NOCl, SnCl₄.2NOCl, SbCl₃.5NOCl, AuCl₃.NOCl, PtCl₄.NOCl. The last two compounds are stable towards heat and may be sublimed, but the majority of these substances decompose on heating with evolution of chlorine or nitric oxide:

CuCl.NOCl = CuCl₂ + NO.

Unsaturated organic compounds also form additive compounds: >C=C< + NOCl → >C(NO)-C(Cl)<.

Amines are decomposed with the formation of chlorides and evolution of nitrogen:

R.NH₂ + NOCl = RCl + N₂ + H₂O.

Sulphur trioxide reacts with nitrosyl chloride to form nitroso-sulphuryl chloride.