Preparation of Hydriodic Acid (HI)

57% Hydriodic acid

A 1.5 litre three-necked flask is charged with a mixture of 480g of iodine and 600 ml of water. The central aperture is fitted with a stopper carrying an efficient mechanical stirrer leading almost to the bottom of the flask, and the smaller apertures respectively with a lead-in tube for hydrogen sulphide extending to well below the surface of the liquid and with an exit tube attached to an inverted funnel just dipping into 5% NaOH. The mixture is vigorously stirred, and a stream of hydrogen sulphide (*) is passed in as fast as it is absorbed. After several hours the liquid assumes a yellow colour (sometimes even colorless) and most of the sulphur sticks together in the form of a hard lump. The sulfur is removed by filtration through a funnel plugged with glass wool (or through a sintered glass funnel). The hydriodic acid is then distilled, and the fraction boiling between 125.5-126.5°C is collected as 57% hydrogen iodide. The yield is 90% of the theorethical.

H2S + I2 => 2 HI + S

* Hydrogen sulfide is generated by dripping HCl on lumps of FeS (made by fusing iron powder with sulfur).

Ref: Vogel, "Practical Organic Chemistry", 3rd Ed.

Anhydrous HI Gas

Hydrogen iodide may be conveniently prepared by allowing a solution of two parts of iodine in one part of hydriodic acid (density 1.7), to drop onto an excess of red phosphorous. The reaction takes place in the cold. When the evolution of gas slackens considerably, the mixture should be gently warmed.

11 parts (by weight) of iodine is placed in a small flask, and 1 part of yellow phosphorous, cut into small pieces and dried, is gradually added. Expect a flash of light and the contents to turn liquid upon the addition. When all the phosphorous has been added, phosphorous tri-iodide is to be separated upon cooling. The product is treated with 1 1/2 parts water, heated gently to produce hydrogen iodide, which is passed over some red phosphorous, that has been moistened with a little water and placed in a U tube. Heating is continued until the liquid just becomes colorless, because if heating is continued further, phosphine and phosphonium iodide are formed, which can cause a powerful explosion. If you require a solution of hydriodic acid (most formulas do), the gas is led through an inverted funnel into a small quantity of cold water. This solution if dilute can be concd by distillation. Bp: 127°C.

Preparation of Pure Anhydrous Solutions of Hydrogen Iodide in Acetic Acid

Org. Proc. Res. Dev., 1 (1), 88-89, 1997.

The presence of molecular iodine in anhydrous solutions of hydrogen iodide in acetic acid gives rise to unstable impurities during the hydriodination of isolated double bonds. This can be overcome by using aqueous hydriodic acid, as the source of hydrogen iodide, from which the iodine has been removed by washing with a solution of an organic soluble ion exchange resin.

In order to develop a practicable synthesis, a procedure for generating anhydrous hydrogen iodide in acetic acid was required. Procedures using molecular iodine (iodine/tetralin at reflux, iodine, and red phosphorus) or compressed hydrogen iodide all proved to be unacceptable. This was because the procedure either was time-consuming or presented safety, handling, or waste management concerns.

All these procedures had one additional and important shortcoming in the context of the proposed chemistry, namely, that traces of iodine, residual during the preparation of the hydrogen iodide, were not readily removable from the resulting solutions produced by passing the gas stream into glacial acetic acid.

The answer was to use analytical grade aqueous hydriodic acid as a readily available and cost effective source of hydrogen iodide. Hydriodic acid of accurately determined concentration was utilised, and all operations were carried out under an argon atmosphere. Traces of molecular iodine were removed by washing with a toluene solution of LA-2 ion exchange resin to produce a colourless and stable aqueous solution. The concentration of hydriodic acid was not affected by the washing process nor was its specific gravity, both of which needed to be accurately determined for the calculation of stoichiometric quantities. The anhydrous acetic acid solutions were prepared by adding the aqueous hydriodic acid to the appropriate quantity of degassed acetic anhydride, with control of the exotherm to below 55°C. The clear and colourless solution was then cooled to 20°C prior to the addition of a solution of alkene in glacial acetic acid. After completion of the required reaction period, the colourless reaction mixture was worked up by vacuum codistillation removal, using toluene, of the majority of the organic and inorganic acids, the product finally being extracted into toluene.


Into an argon-purged separation vessel fitted with a mechanical stirrer is placed hydriodic acid (2.165 L, specific gravity 1.91, 65.0% w/w). A solution of Amberlite LA-2 (0.395 kg) in toluene (5.0 L) is then added to the vessel, and the agitator is used to mix the layers for 2 min. After the layers are allowed to separate, the colourless hydriodic acid layer is run into an argon-purged holding vessel prior to returning to the separator for a single wash with a quantity of degassed toluene. For solutions heavily contaminated with molecular iodine, a second wash with the LA-2 resin solution is required.

Into an argon-purged reaction vessel is then placed acetic anhydride (6.94 L, 99.7%, 73.33 mol) which is vacuum degassed. Washed hydriodic acid (1.973 L, 19.15 mol of HI, 73.33 mol of H2O) is added to the mechanically stirred solution at such a rate that the temperature is maintained below 55°C by the use of external water cooling. If the temperature is allowed to rise above this limit, there is some loss of water vapour by entrainment, and this results in incomplete hydrolysis of the acetic anhydride.

The mixture is stirred for a further 60 min after completion of the addition of the aqueous acid and is then cooled to 20°C prior to the addition of a vacuum-degassed solution of alkene (4.822 mol) in glacial acetic acid (2.0 L) over a period of 10 min.

After completion of the addition, the mixture is stirred for a further 16 h prior to removal of the majority of the acetic acid by vacuum codistillation with 10 volumes of toluene (50 mmHg, <50°C). The dark residue is dissolved in toluene (14.0 L) and then transferred to a separating vessel followed by washing with a 5% solution of sodium thiosulphate (2.0 L) and then deionised water. The thiosulphate wash is first back-washed with a small quantity of toluene, which is combined with the main solution of product.

The organic solution is dried over magnesium sulphate and filtered through a short bed of 100-200 mesh Florisil prior to removal of the toluene under reduced pressure, to leave the product iodoalkene as a colourless to very pale yellow oil. Yield range: 90-97%.

Flaky solid iodine of 40 g was dissolved in tetrahydronaphthalene (Tetralin) of 160 g charged in a flask of 500 ml at 40°C to prepare a tetrahydronaphthalene solution of iodine. A flask of 500 ml was charged with tetrahydronaphthalene of 40 g and heated to 200°C. while stirring. The iodine solution prepared above was continuously added thereto over a period of 2 hours while maintaining the above temperature to react them. Crude hydrogen iodide gas generated as the reaction went on was introduced into a 10% sodium hydroxide aqueous solution of 1 liter to absorb the whole amount thereof. A weight change in this aqueous solution was measured with the lapse of time, and the end point of the first reaction was set at the point where the change thereof was not observed. The yield of the crude hydrogen iodide was 94.6%, and the purity thereof was 99.5% or more.

Ref:US Patent 5,693,306