Replacement of NH2 in aromatic compounds by iodine

The preparation of / -diiodobenzene given as an example above showed that iodine can be very easily substituted for a diazonium group by means of hydriodic acid without addition of a copper(i) salt. A little urea is added to the diazotized solution (to remove residual HN02), which is usually prepared in hydrochloric or sulfuric acid, followed by a cold iodide solution (1-2 moles of alkali iodide per mole of diazotized amine) or the solution of the diazonium salt in mineral acid is added to a cold iodide solution. Evolution of nitrogen ceases after storage for several hours or, if necessary, subsequent warming on a wather-bath. Phenols can be expected as by-products and may be removed by basification before isolation of the aryl iodide the latter is obtained then by distillation in steam, extraction in ether, CHC13, benzene, or toluene, or filtration. Biaryls and azo compounds are not formed, because the mechanism 

The usual experimental procedure is exemplified in the preparation of iodobenzene from aniline detailed in Organic Syntheses.1221 

Acid-sensitive diazotized amines are treated with acidified iodide solution in the presence of a water-immiscible solvent, e.g., CHC13, which removes the aryl iodide as it is formed 1220,1222 

Diazonium salt solutions that have been obtained from weakly basic amines in concentrated H2S04 with nitrosylsulfuric acid and 85 % phosphoric acid give good yields of the derived aryl iodides when treated with cold aqueous KI for example, the preparation of 1,2,3-triiodo-5-nitrobenzene from 2,6-diiodo-4-nitroaniline is described in detail in Organic Syntheses 1223 and m-iodonitrobenzene, l,2-diiodo-4-nitrobenzene, l,4-diiodo-2-nitrobenzene, and 1,2,4-tri-iodo-3-nitrobenzene have been obtained analogously.1224 

Addition of copper is sometimes recommended for instance, adding 1.2 moles of KI and 1 g of copper bronze to a diazonium solution from 1 mole of p-amino-phenol in dilute sulfuric acid give 69-72% of /Modophenol.1230 The technique for addition of iodine is illustrated in the following example. 

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