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Synthesis of Iodobenzene

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Iodobenzene by Sandmeyer Reaction on Aniline

In a 3- or 5-gallon stoneware crock are placed 950 mL (1130 g., 11.7 moles) of concentrated hydrochloric acid (sp. gr. 1.19), 950 mL of water, 200 g. (196 mL, 2.15 moles) of aniline, and 2 kg. of ice (Note 1). The mixture is agitated by a mechanical stirrer, and, as soon as the temperature drops below 5C, a chilled solution of 156 g. (2.26 moles) of sodium nitrite in a measured volume (7001000 mL) of water is introduced fairly rapidly from a separatory funnel, the stem of which projects below the surface of the reaction mixture. The addition should not be fast enough to cause the temperature to rise to 10C or to cause evolution of oxides of nitrogen. The last 5% of the nitrite solution is added more slowly, and the reaction mixture is tested with starch-iodide paper at intervals until an excess of nitrous acid is indicated.

Stirring is continued for ten minutes, and if necessary the solution is filtered rapidly through a loose cotton plug in a large funnel. An aqueous solution of 358 g. (2.16 moles) of potassium iodide is added and the reaction mixture allowed to stand overnight. The mixture is transferred to a large flask (or two smaller flasks) and heated on a steam bath, using an air-cooled reflux condenser, until no more gas is evolved, then allowed to cool and stand undisturbed until the heavy organic layer has settled thoroughly. A large part of the upper aqueous layer is siphoned off, and discarded (Note 2). The residual aqueous and organic layers are made alkaline by the cautious addition of strong sodium hydroxide solution (100125g of solid technical sodium hydroxide is usually required) and steam-distilled at once. The last one-third of the steam distillate is collected separately and combined with the aqueous layer separated from the earlier portions of the distillate. This mixture is acidified with 510 mL of concentrated sulfuric acid and steam-distilled again. The iodobenzene from this operation is combined with the main portion and dried with 1015 g. of calcium chloride (Note 3) and (Note 4). Distillation under reduced pressure gives 327335g (74-76% of theory) of iodobenzene, bp 7778C/20 mm. or 6364C/8 mm. (Note 5).


Notes

  1. If more ice is used a portion remains unmelted after the diazotization is completed.
  2. If a good separation has been made not more than 12g of iodobenzene is lost with the upper layer.
  3. An appreciable amount of iodobenzene is retained by the solid calcium chloride. By treating the spent drying agent with water 812g of iodobenzene can be recovered.
  4. The crude iodobenzene weighs 350355g (80% of theory) and is pure enough for many purposes without redistillation.
  5. If the distillation is carried too far, the distillate will be colored.

Iodobenzene by Direct Iodination of Benzene

In a 1-L three-necked flask fitted with mechanical stirrer, reflux condenser, and separatory funnel, are placed 381g (1.5 moles) of iodine and 400 g. (455 mL, 5.1 moles) of benzene. The mixture is heated to about 50 on a water bath, and 275 mL (6.15 moles) of nitric acid (sp. gr. 1.50) is added slowly from the separatory funnel; the time required for the addition should be about one and one-quarter hours. A copious evolution of oxides of nitrogen takes place, and the gases are carried off from the upper end of the condenser to an open window or hood, or absorbed by means of a gas trap. The reaction proceeds smoothly (Note 1), and the temperature rises slowly without the application of heat until the mixture boils gently. When all the nitric acid has been added, the solution is refluxed for about fifteen minutes. If iodine still remains, more nitric acid should be added slowly to the warm solution until the purple color of the iodine has been discharged and the solution becomes brownish red.

The lower reddish oily layer is separated, mixed with an equal volume of 10 per cent sodium hydroxide solution, and steam-distilled from a 2-L flask until no more oil passes over. Towards the end of the distillation a yellow solid begins to collect in the receiver; this consists of nitro compounds, which are removed by vigorously stirring the oil for about three hours with 20 mL of concentrated hydrochloric acid, 300 mL of water, and 200g of iron filings in a 2-L flask connected with a reflux condenser.

The mixture is allowed to cool and is then filtered. The filtrate is rendered distinctly acid to Congo red with hydrochloric acid and again distilled with steam. The oil so obtained is separated and distilled under normal pressure with the use of a fractionating column (Note 2). The fraction boiling at 180190C is redistilled, and the pure compound is collected at 184186C. The yield is 523531g (8687% of theory) (Note 3).


Notes

  1. The reaction proceeds smoothly without stirring; however, the time of addition is decreased somewhat by stirring. Ordinary rubber stoppers may be used; although they are somewhat attacked, this is not sufficiently serious to warrant any special apparatus. In very large runs it may be desirable to use stoppers made from asbestos paper and water glass. Rubber stoppers have been used in a run five times the size of that described.
  2. A good separation is obtained by means of a 500-mL modified Claisen flask without the use of diminished pressure.
  3. Iodobenzene prepared by this procedure may contain traces of nitro compounds. The test for these is to reduce a sample with stannous chloride (or tin) and hydrochloric acid, and treat the resulting acid solution in the cold with a solution of sodium nitrite. If a phenolic odor is obtained on boiling the solution, nitro compounds are still present. A more delicate test can be made by adding the reduced solution, after treatment with sodium nitrite, to an alkaline solution of β-naphthol; an azo dye is formed if nitro compounds were originally present.

References

  1. Organic Syntheses, Coll. Vol. 2, p. 351
  2. Organic Syntheses, Coll. Vol. 1, p. 323