Reactive polyhalides

Polyhalide salts are prepared in situ in the bromination of less reactive aromatic substrates. Thus, tetra- -butylammonium bromide(TBAB) functions as a catalyst in the bromination of benzene, toluene and even benzyl bromides and a-bromoethylbenzene537 (both tend to polymerize in presence of normal Friedel-Crafts catalysts). Tetra-ethylammonium chloride in CH2Cl2/methanol catalyzes the bromination of anilines538. Improved selectivity is found in the chlorination of toluene catalyzed by onium chloride salts alone or combined with SnCl4539. 

The choice of an appropriate solvent as the reaction medium is of primary importance and usually involves a compromise among various unfavorable factors. Because of the high chemical reactivity of the halogens and of the polyhalides, solvents which are easily susceptible to halo-genation or which effect solvolysis reactions are to be avoided. This, of course, precludes the use of aqueous solutions for many syntheses, with the exception of those of some polyiodides since iodine is fairly unreactive with water. 

Partial replacement. BrFj added to a vigorously stirred soln. or startg. polyhalide in Freon 113 at 20-5" in the presence of a little SbClj, and worked up after ca. 1.5 h product. Y 90%. A Teflon reactor, fitted with a nickel stirrer and quartz dropping funnel, was used. The method is useful for halogen exchange in polychloroalkanes of low reactivity. F.e.s. A.V. Kartashov et al., Zh. Org. Khim. 24, 2522-5 (1988). 

Reactivity of the elements Hydrogen halides Interhalogens Polyhalogen cations Polyhalide anions Oxides and oxoacids Aqueous solution chemistry... 

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