Chromyl Chloride, CrO₂Cl₂

Chromyl Chloride, CrO₂Cl₂, sometimes termed chlorochromic acid, is readily prepared by distilling a mixture of fused potassium chromate or dichromate and sodium chloride with concentrated sulphuric acid:

3K₂CrO₄ + 6NaCl + 12H₂SO₄ = 6KHSO₄ + 6NaHSO₄ + 3CrO₂Cl₂ + 6H₂O,

and, in order to remove free chlorine, repeatedly fractionating the distillate over mercury in a current of carbon dioxide. It is more expeditiously obtained, however, by dissolving chromic anhydride, CrO₃, in concentrated hydrochloric acid, and adding excess of concentrated sulphuric acid, when chromyl chloride is formed, which sinks to the bottom and may readily be separated. After aspirating dry air through the liquid in order to remove dissolved hydrochloric acid, it is subjected, as before, to fractional distillation. Other methods of formation are: (a) by heating together two equivalents of chromic chloride and three of chromic anhydride, or equal parts of the latter and ferric chloride; (b) by passing hydrogen chloride over chromic anhydride; (c) by heating together phosphorus pentachloride and either chromic anhydride or potassium dichromate; (d) by the action of chlorine or hydrochloric acid upon the sesquioxide; (e) by heating together dry powdered chromic acid, acetyl chloride, and a little glacial acetic acid in carbon tetrachloride solution. It is also produced by heating a chlorochromate with sulphuric acid, or by heating the oxychloride (CrO₂)₅Cl₆.

Chromyl chloride is a dark red, mobile liquid, of mean density 1.9587 at 0° C. It boils at 116.63° C. under 760 mm. pressure, and the solid melts at -96.5±0.5° C. It has been suggested for use as an ebulliscopic solvent, and gives a constant of 55 with chromic anhydride. Its yellowish-red vapour, which absorbs all the light from a luminous source except a narrow band in the red, has a density at temperatures up to 200° C. of 5.35 (air = l) corresponding to the formula CrO₂Cl₂.

Chromyl chloride fumes in air owing to the fact that it is decomposed by water into chromic and hydrochloric acids. The fumes when introduced into a Bunsen flame exhibit a characteristic spectrum. When sufficiently purified by distillation, and free from access to moisture, chromyl chloride may be kept unchanged for a considerable time, though on very long standing some dark coloured solid - probably a polymeride - is deposited. This tendency to polymerise is also suggested by the results of cryoscopic molecular weight determinations in carbon tetrachloride, tin tetrabromide, or antimony pentachloride.

When heated in a sealed tube at 180° C. for three hours, chromyl chloride is converted to chromium chlorochromate (trichromyl chloride) and chlorine, while by passing through a tube heated at 400° C., a grey or black deposit is obtained, consisting largely of the magnetic oxide, Cr₅O₉; at red heat the sesquioxide is formed. It readily parts with oxygen and chlorine, and is therefore an energetic oxidising and chlorinating agent, for example, towards sulphur, hydrogen sulphide, phosphorus (with explosion), hydrogen phosphide, phosphorus trichloride and oxvchloride, alcohol, turpentine, benzene and other hydrocarbons, fats, camphor, caoutchouc, and organic substances in general. In dry ammonia it burns, forming ammonium chloride and chromium dioxide. When ammonia gas is passed into a solution of chromyl chloride in acetic acid or chloroform, a slightly soluble brown substance, having the composition Cr₃O₆(ONH₄)₂, is formed. A mixture of gaseous chromyl chloride and dry chlorine passed over carbon heated to redness yields chromic chloride in a well-crystallised condition. The course of the reaction appears to be:

2CrO₂Cl₂ + Cl₂ + 4C = 2CrCl₃ + 4CO.

Chromyl chloride absorbs chlorine readily; it dissolves iodine, forming a solution which on heating yields trichromyl chloride, Cr₃O₆Cl₂, and iodine monochloride; with an aqueous solution of potassium chloride, potassium chlorochromate is produced.

Neither bromine nor iodine forms compounds analogous to chromyl chloride under any of the conditions given above, consequently the presence of a chromate in the aqueous solution of the distillate obtained by distilling a chloride, bromide, or iodide, or a mixture of these, with potassium dichromate and concentrated sulphuric acid, is sufficient evidence of the presence of a chloride in the mixture investigated.