Carbon Tetrachloride, CCl₄

Carbon tetrachloride, CCl₄, is the final chlorination product of methane, but it is generally prepared by the action of chlorine on carbon disulphide through the catalytic agency of a chlorine-carrier. Thus Hofmann dissolved antimony trichloride in carbon disulphide, and passed dry chlorine through the solution. The trichloride became pentachloride, which then reacted with the carbon disulphide, thus:

CS₂ + 2SbCl₅ = CCl₄ + 2SbCl₃ + 2S.

Other catalysts which may be employed are chloride of bromine, chloride of iodine, phosphorus pentachloride, molybdenum pentachloride, and aluminium chloride.

Carbon tetrachloride is also prepared in several ways by the action of chlorine on carbon in the electric furnace.

Carbon tetrachloride is a colourless liquid which smells like chloroform; it does not mix with water, though it appears to form an unstable hydrate; it dissolves in many organic liquids, and is itself a useful solvent. The properties of mixtures of ethyl alcohol, carbon tetrachloride, and water have been examined by T. H. Hill. Its density at 0° C. is 1.5947, and at 32.5° C. is 1.56834; its boiling-point is 76.65° C. at 760 mm. pressure. The critical temperature is 282.1° C., and the critical pressure 57.57 atm. The specific heat of liquid carbon tetrachloride rises from 0.2010 at 0° C. to 0.2031 at 70° C.; that of the vapour falls from 0.140 at 0° C. to 0.115 at 70° C. Solid carbon tetrachloride is trimorphous, the melting-points of the three modifications being -28.6° C., -23.77° C., -21.2° C. The depression constant, when camphor or ethyl benzoate is used as solute, is found to be 47 or 48.

When the vapour of carbon tetrachloride is passed through a red-hot tube chlorine is split off and the compounds C₂Cl₄ and C₂Cl₆ result. From the latter compound, hexachlorethane, trichloracetic acid is formed by aqueous hydrolysis in the presence of sunlight, and this is reduced by nascent hydrogen to acetic acid. Since carbon tetrachloride can be obtained from its elements by way of carbon disulphide, this compound marks a stage in the synthesis of acetic acid.

Chemically, carbon tetrachloride is the chloride of the hypothetical orthocarbonic acid, C(OH)₄, just as the analogous silicon compound is the chloride of orthosilicic acid, Si(OH)₄. Although carbon tetrachloride, unlike the silicon halide and acid chlorides in general, is not attacked by water at atmospheric temperature, this halide is capable of some reactions which reveal its relation to carbonic acid. When heated with oxygen and water under pressure at 250° C. it yields phosgene gas, COCl₂, and carbon dioxide, CO₂, and a similar change is brought about by certain acidic oxides, e.g.:

2CCl₄ + P₂O₅ = COCl₂ + CO₂ + 2POCl₃
3CCl₄ + 2P₂O₅ = 3CO₂ + 4POCl₃
CCl₄ + 2SO₃ = COCl₂ + S₂O₅Cl₂.

When heated with alcoholic potash carbon tetrachloride is completely hydrolysed, yielding carbonate and chloride, thus:

CCl₄ + 6KOH = K₂CO₃ + 4KCl + 3H₂O.

It reacts with acetylene, with liberation of finely divided carbon, thus:

CCl₄ + 2C₂H₂ = 5C + 4HCl.

Its vapour readily chlorinates various oxides, and may on this account be used in mineral analysis.