Org. Synth. 1951, 31, 62
DOI: 10.15227/orgsyn.031.0062
HEXAMETHYLENE DIISOCYANATE
[Isocyanic acid, hexamethylene ester]
Submitted by Mark W. Farlow
1
Checked by R. L. Shriner and Robert C. Johnson.
1. Procedure
Caution! Hexamethylenediamine, hexamethylene diisocyanate, and phosgene are highly toxic. Exposure to vapors or solutions containing these materials should be avoided. All operations should be conducted in a hood.
A. Hexamethylenediammonium chloride. To a solution of 116 g. (1.0 mole) of hexamethylenediamine (Note 1) in 145 ml. of methanol in a 1-l. beaker is added slowly from a dropping funnel 175 ml. of concentrated hydrochloric acid (sp. gr. 1.19). The mixture is well stirred during the addition and cooled externally to keep the contents below 30°. The hexamethylenediammonium chloride is then precipitated by adding the solution slowly with stirring to approximately 2 l. of acetone. The precipitate is collected on a Büchner funnel, washed with 100 ml. of cold acetone, and dried in a vacuum oven at 75° for 12–18 hours (Note 2). The yield of dry product amounts to 170–187 g. (90–99%), m.p. 243–246° (Note 3).
B. Hexamethylene diisocyanate. A suspension of 94.5 g. (0.50 mole) of finely powdered hexamethylenediammonium chloride in 500 ml. of anhydrous redistilled amylbenzene (or tetralin) (Note 4) is prepared in a 1-l. three-necked flask fitted with an efficient mechanical stirrer (Note 5), a water-cooled reflux condenser, a thermometer, and a phosgene inlet tube (Note 6) extending well below the surface of the suspension. Stirring is started, the mixture is heated to 180–185° (Note 7), and gaseous chlorine-free phosgene (Note 8) is delivered to the mixture at a rate of about 33 g. (0.33 mole) per hour. Hydrogen chloride and excess phosgene escape through the condenser. The temperature is carefully maintained between 180° and 185°; after 8–15 hours (Note 9) solution of the hexamethylenediammonium chloride is essentially complete and hydrogen chloride is no longer evolved (Note 10). The reaction mixture is then filtered through a suction filter, and the filtrate is distilled at reduced pressure through a fractionating column, giving amylbenzene, b.p. 65–75° /10 mm. (Note 11), and 70–80 g. (84–95%) of hexamethylene diisocyanate boiling at 120–125° /10 mm. (92–96° /1 mm., 108–111° /5 mm.); nD20 1.4585; d420 1.0528 (Note 12).
2. Notes
1.
Hexamethylenediamine may be obtained from the Polychemicals Department, E. I. du Pont de Nemours and Company, Inc., Wilmington, Delaware. Usually the commercial product is a 70% aqueous solution from which the water may be distilled at atmospheric pressure. The residue is suitable for preparing the dihydrochloride.
2.
Thorough drying of the hydrochloride is essential to the success of the next step. Overnight drying in a vacuum
oven at 70–100° is effective. The dry salt is not appreciably hygroscopic, but it should be preserved in a
well-stoppered bottle until used.
3.
This salt shows a marked tendency to sublime at and above 200°.
4.
It is preferable to add the
hexamethylenediaommnium chloride in three portions. This modification facilitates stirring and prevents clumping of the solid.
2Commercial
amylbenzene is dried by distillation, and the fraction boiling at
184–194° is collected. This solvent has been withdrawn from the market (1950) and hence
tetralin may be substituted. Commercial
tetralin is washed with
ferrous sulfate solution and distilled; the fraction boiling at
202–206° is used.
o-Dichlorobenzene also has been recommended as a solvent.
3 When the latter was employed,
2 it was found advisable to use recovered material, since it contained some component that acted as a solvent for the hydrochloride. The
phosgene was absorbed more rapidly, and the reaction was completed in only 9 hours.
5.
Efficient agitation of the gas-liquid-solid reaction mixture is conducive to a high rate of reaction. The use of a reaction flask modified with creases
4 has given good results.
6.
The end of the inlet tube should have a very coarse fritted glass disk in order to promote rapid reaction. A tube with a bulb in which many fine holes have been blown may also be used, but the reaction time is longer. If the inlet tube becomes clogged, it may be cleaned quickly by removing it from the reaction flask and dipping in warm
cresol, which dissolves any
polyhexamethylene urea that may form. The tube is then rinsed with
amylbenzene or
tetralin and replaced in the reaction flask.
7.
The temperature of the reaction mixture should be maintained between 180° and 185° in order to obtain as rapid a reaction as possible. Higher temperatures lead to the formation of
polyhexamethylene urea. A run carried out at the boiling point of
tetralin (
206°) gave an
84% yield of polymer.
8.
Phosgene is available from the Niagara Chlorine Products Co., Inc., Lockport, New York, or the Matheson Co., Inc., Rutherford, New Jersey. When
phosgene containing small amounts of
chlorine is used, the reaction appears to proceed normally but the product and recovered solvent are contaminated with chlorine-containing impurities.
Chlorine in
phosgene can be detected by bubbling a stream of the gas rapidly through clean
mercury.
Chlorine reacts with and discolors the
mercury, whereas pure
phosgene leaves the
mercury unchanged. If
chlorine is present, it may be removed by bubbling the
phosgene through two
wash bottles containing cottonseed oil.
9.
The time required for complete reaction is dependent on the reaction temperature, on the design of the
phosgene inlet tube, on the efficiency of agitation, and on the rate of
phosgene addition. It is important that the reaction be continued until practically all the
hexamethylenediammonium chloride has disappeared. If unreacted amine salt is present, it has a tendency to sublime with the diisocyanate during distillation.
10.
When moist air is blown through a glass tube held at the end of the condenser, across the current of
phosgene containing
hydrogen chloride, the fogging typical of
hydrogen chloride gas in a moist atmosphere is produced. Pure
phosgene gives no visible effect under similar conditions.
11.
The recovered solvent is suitable for succeeding preparations. If
tetralin is used as the reaction medium, it is recovered as the low-boiling fraction, b.p.
60–70°/8 mm.12.
If the product contains
chlorine as indicated by the alcoholic
silver nitrate test, it may be purified by adding a small amount of anhydrous
calcium oxide (0.5 g. per 50 g. of product) and redistilling under reduced pressure.
3. Discussion
Hexamethylene diisocyanate has been prepared by the action of
phosgene on
hexamethylenediammonium chloride5 or on
hexamethylenediammonium carbonate.
6 Metal chlorides such as those of
cobalt,
iron,
mercury, or
zinc have been stated to promote the reaction.
3 It has been claimed
7 that the vapor-phase reaction of an
hexamethylenediamine-toluene solution with
phosgene at 280–300° affords an
80% yield of the diisocyanate.
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
hexamethylenediaommnium chloride
polyhexamethylene urea
hexamethylenediamine-toluene
hydrogen chloride,
hydrochloric acid (7647-01-0)
methanol (67-56-1)
iron (7439-89-6)
silver nitrate (7761-88-8)
ferrous sulfate (13463-43-9)
mercury (7439-97-6)
acetone (67-64-1)
chlorine (7782-50-5)
zinc (7440-66-6)
phosgene (75-44-5)
cresol (95-48-7)
calcium oxide
Tetralin (119-64-2)
Amylbenzene (538-68-1)
cobalt (7440-48-4)
Hexamethylene diisocyanate,
Isocyanic acid, hexamethylene ester (822-06-0)
hexamethylenediamine (124-09-4)
hexamethylenediammonium chloride
hexamethylenediammonium carbonate
o-dichlorobenzene (95-50-1)
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