Through Exchange of Halogen

Halogen exchange reactions may be used to prepare tetrahalotellurates(IV) that are not accessible through reactions of aryl tellurium trihalides and onium halides. Treatment of tetrachloro- or bromotrichlorotellurates with dilute hydrobromic acid produces the tetrabromotellurates. The conversion of bromotellurates to tetrachlorotellurates requires concentrated hydrochloric acid. Chlorine and bromine are easily replaced by iodine when the tellurates are reacted with excess potassium iodide in dilute hydrochloric acid. The iodotellurates cannot be converted to the bromo- or chlorotellurates  

Onium Aryltetrabromotellurates(IV) 2 mmol of the phosphonium or telluronium chlorotellurate are dissolved in 30 ml of tetrahydrofuran (or a suspension of 2 mmol of the tropylium compound is prepared) and 15 m/of 10% hydrobromic acid are added dropwise. The tropylium aryltetrabromotellurales precipitate whereas the phosphonium and telluronium aryltetrabromotell urates are obtained on evaporation of the tetrahydrofuran. 

The following compounds were prepared in this manner  

Onium Aryltetraiodotellurates(IV) The chloro- or bromotellurate is suspended in 1 molar hydrochloric acid and an excess of aqueous potassium iodide solution is added with stirring. The precipitated tetraiodotellu-rate is filtered and dried yield 100%. 

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