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Thallium chlorobromides

The following intermediate chloro-bromides have been described: -

Type TlX2Type Tl2X3

Type TlX2. - (a) The compound Tl3Cl2Br4 may be prepared by treating thallous chloride in solution with excess of bromine, and concentrating the solution of thallic chlorobromide TlClBr2 over sulphuric acid in vacuo. A syrupy liquid is produced which on further concentration begins to decompose, and yellow crystals of Tl3Cl2Br4 are deposited. Any thallic hydroxide produced is separated by decantation while there is still a little liquid left, and the crystals are then left over sulphuric acid until they are dry. The compound Tl3Cl2Br4 forms sulphur-yellow, orthorhombic (?) prisms, and melts at 165° to a yellow liquid. It is decomposed by water with the production of impure Tl4Cl3Br3, which may be purified by recrystallising it from hot water.

(b) The compound Tl3Cl4Br2 is made by the method outlined for Tl3Cl2Br4, but the starting materials are thallous bromide and chlorine. It resembles the preceding compound, and is decomposed by water with the production of Tl4Cl3Br3.

(c) The compound TlClBr is obtained by heating thallous chloride with bromine.

(d) The compound Tl2Br3Cl was obtained by Cushman by the action of aqueous thallic bromide on the compound Tl4Br3Cl3. It crystallises in yellow needles.

Type Tl2X3. - (e) One method for the preparation of the compound Tl4Cl3Br3 has been given under (a) above. It is also said by Cushman to be produced by boiling a solution of thallic chloride with thallous bromide and cooling, or by boiling a solution of thallic bromide with thallous chloride. It is also produced when a limited quantity of bromine is added to thallous chloride suspended in cold water, the yellow solid produced dissolved in boiling water, and the filtered solution slowly cooled. The filtrate from the deposited crystals, when concentrated and cooled, deposits more of the compound Tl4Cl3Br3, in six-sided plates, but mixed with needles of the compound Tl4Cl4Br2.

(f) One method, due to Thomas, for the production of Tl4Cl4Br2 has just been mentioned, but it is difficult to obtain the compound free from admixed Tl4Cl3Br3. R. J. Meyer and Cushman have also described processes for the preparation of Tl4Cl4Br2 from thallous chloride, bromine, and water.

(g) The preparation of Tl4Cl2Br4 from thallous bromide, chlorine, and water has been described by R. J. Meyer.

Each of the compounds Tl4Cl3Br3, Tl4Cl2Br4, and Tl4Cl4Br2 forms dark orange-yellow crystals, sparingly soluble in water. With rise of temperature the crystals rapidly become deep red in colour. The two methods given by Cushman for the preparation of Tl4Cl3Br3 are said by that author to yield stereo-isomeric forms of the compound, but at present this cannot be regarded as proved.

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