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.
Experimental:
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
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