Carbon Oxychloride, COCl₂

The name phosgene was given to this gas by J. Davy, who in 1811 observed its generation by the action of light upon a mixture in equal volumes of carbon monoxide and chlorine. This phenomenon is analogous to that of the union of hydrogen and chlorine under the influence of light; the union of carbon monoxide and chlorine has, however, this advantage, that it is accompanied by a diminution of volume at constant pressure, or of pressure at constant volume; consequently the course of the reaction can be conveniently traced.

It was observed by Bunsen and Roscoe that when a mixture of hydrogen and chlorine is exposed to light the amount of hydrogen chloride produced per unit time increases for a certain period, after which the rate of combination becomes proportional to the amount of gas remaining uncombined; and to the action during this preliminary period these observers gave the name " photochemical induction."

Dyson and Harden observed a similar phenomenon with carbon monoxide and chlorine, and attributed it to the specific action of light on chlorine; and Chapman and Gee came to a similar conclusion, though the interpretation they gave of the induction process is quite different from that of Bunsen and Roscoe. These observers have found that small quantities of nitric oxide, ozone, and nitrogen chloride inhibit the photochemical action, i.e. extend the period of so-called "induction," during which this action is slow, and almost entirely prevent it. Thus they attribute photochemical action to the use chlorine can make of absorbed and transformed light energy to effect union with carbon monoxide or hydrogen, before such energy is degraded to the form of ineffective heat. The inhibiting action of extraneous gases is then due to the hastening of the process of energy degradation, and the consequent loss of efficiency by the chlorine.

Carbonyl chloride is best prepared by passing its constituent gases separately into a large glass balloon exposed to sunlight, and thence through a second balloon similarly exposed. The chlorine should be in excess, and may afterwards be absorbed by means of metallic antimony; whilst the carbonyl chloride may be purified by liquefaction in a freezing mixture. Carbon monoxide and chlorine may also be made to combine through the influence of electric sparks, animal charcoal, or spongy platinum.

Carbonyl chloride results from the action of acidic oxides such as P₂O₅ and SO₃ or H₂S₂O₇ upon carbon tetrachloride. Sulphur trioxide reacts at 100° C., phosphoric oxide at 200° C.

CCl₄ + 2SO₃ = COCl₂ + S₂O₅Cl₂
2CCl₄ + P₂O₆ = COCl₂ + 2POCl₃ + CO₂.

Carbonyl chloride may also be prepared by the oxidation of chloroform by chromic acid, when 20 parts of chloroform, 400 of sulphuric acid, and 50 of potassium dichromate are heated together on the water-bath:

2CHCl₃ + K₂Cr₂O₇ + 4H₂SO₄ = 2COCl₂ + K₂SO₄ + Cr₂(SO₄)₃ + 5H₂O + Cl₂.

Another method of formation of carbonyl chloride, which takes place in the electric furnace, is represented by the following equation:

2CaO + 2CaCl₂ + 10C = 4CaC₂ + 2COCl₂.

Carbonyl chloride is a colourless gas with a pungent taste and suffocating smell. Its density under normal conditions is 3.505 (theory requires 3.4168). It is easily condensed to a liquid, having a density of 1.432 at 0° C., and boiling at 8.2° C. under 756 mm. pressure; the solid melts at -118° C. The heat of formation from amorphous carbon at constant pressure [C,O,Cl₂] is, according to Thomsen, 55,140 calories, and from diamond 44,100 calories (Berthelot). The gas dissolves in water and alcohol, undergoing decomposition.

The dissociation of carbonyl chloride according to the reaction COCl₂ ⇔ CO + Cl₂ has been studied by Bodenstein and Durrant, who passed the gas or an equimolecular mixture of carbon monoxide and chlorine through a heated tube, and analysed the issuing gas. Thus they found the following percentages decomposed at the given temperatures:

Temperature ° C.	503	553	603	800
Per cent, decomposed	67	80	91	100

Thence they calculated the heat of formation of carbonyl chloride from carbon monoxide and chlorine to be about 23,000 calories, whereas Berthelot9 obtained the value 18,800 calories, and Thomsen 26,140 calories. According to Coehn and Becker the dissociation equilibrium of carbonyl chloride is affected by ultraviolet light.

Carbonyl chloride is the chloride of carbonic acid, and as such reacts with water thus:

COCl₂ + H₂O = H₂CO₃ + 2HCl,

the carbonic acid then decomposing into carbon dioxide and water.

With ammonia there is produced carbamide or urea,

COCl₂ + 4NH₃ = CO(NH₂)₂ + 2NH₄Cl,

and with hydrogen sulphide carbonyl sulphide, thus:

COCl₂ + H₂S = COS + 2HCl,

whilst alcohol yields chlorocarbonic and carbonic esters:

COClOC₂H₅ and CO(OC₂H₆)₂.

Certain metals when heated in carbonyl chloride form chlorides, liberating CO, e.g.: COCl₂ + Zn = ZnCl₂ + CO; and numerous oxides are converted into chlorides when heated in a stream of carbonyl chloride. Mineral sulphides are also converted into chlorides by the reaction:

MS + COCl₂ = MCl₂ + COS.

Natural phosphates and silicates also yield chlorides when heated with carbonyl chloride vapour.

Carbonyl chloride is employed in the manufacture of certain synthetic dye-stuffs, e.g. it reacts with dimethylaniline in the presence of aluminium chloride to form tetramethyldiamidobenzophenone:

COCl₂ +2HC₆H₄N(CH₃)₂ = + 2HCl