Replacement of halogen by iodine, bromine, or chlorine

Iodine is most easily introduced in place of another halogen in an organic halide RX by allowing the latter to react with an inorganic iodide, especially Nal or, less often, KI or Cal2. 

The rate at which the equilibrium (a) is established depends greatly on the solvent it is particularly high in acetone, which has the further advantage that it dissolves Nal very well (1.29 mole/1) but NaBr and particularly NaCl (5.5 x 10-6 mole/1) only very slightly the solubility of Nal in dry acetone at room temperature is about 20 times that of KI. a-Halo carbonyl compounds, and allyl and benzyl halides are particularly reactive, bromine being more easily replaced than chlorine. Primary alkyl halides react more readily than secondary or tertiary ones.822,823 

Substitution of I for X in aromatic compounds by means of Nal is possible only when the X is activated by electron-attracting substituents such as o- 

Olah and A. Pavlath, Acta Chim. Acad. Sci. Hung., 3, 199 (1953). 

Reaction of RX with Nal is normally very smooth but side reactions may be expected in some cases. Particularly when the iodine is very mobile, the HI produced may act as reducing agent (b), so that it is advisable to remove the iodine by thiosulfate or dilute sodium hydroxide solution during working up instead of by hydrogen sulfite. 

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