Methyl isocyanate

Methyl isocyanate (MIC) is an organic compound with the molecular formula C₂H₃NO, arranged as H₃C-N=C=O. Synonyms are isocyanatomethane, methyl carbylamine, and MIC. It was discovered in 1888 as an ester of isocyanic acid. Methyl isocyanate is an intermediate chemical in the production of carbamate pesticides (such as carbaryl, carbofuran, methomyl, and aldicarb). It has also been used in the production of rubbers and adhesives. As a highly toxic and irritating material, it is hazardous to human health, and was involved in the Bhopal disaster which killed nearly 3,800 people.^[1]

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Physical properties

Methyl isocyanate (MIC) is a clear, colorless, lachrymatory, sharp smelling liquid. It is highly flammable, boils at 39.1 °C and has a low flash point.^[2] Methyl isocyanate is soluble in water to 6-10 parts per 100 parts, but it reacts with the water (see Reactions).

Manufacture

The methyl isocyanate is obtained by treating the MCC with a tertiary amine (e. g.: dimethylaniline, pyridine^[3] or by separating it by using distillation techniques.^[4]

Methyl isocyanate is also manufactured from N-methylformamide and air. In the latter process it is immediately consumed in a closed-loop process to make methomyl.^[5] Other manufacturing methods have been reported.^[6][7]

Reactions

Methyl isocyanate reacts readily with many substances that contain N-H or O-H groups and some other compounds. It also reacts with itself to form a trimer or higher molecular weight polymers.

Methyl isocyanate reacts with water to form 1,3-dimethylurea and carbon dioxide with the evolution of heat (325 calories per gram of MIC that reacts).

At 25 °C, in excess water, one-half of the MIC is consumed in 9 minutes;^[8] if the heat is not efficiently removed from the mixture the rate of the reaction will increase and rapidly cause the MIC to boil. If MIC is in excess, 1,3,5-trimethylbiuret is formed along with carbon dioxide.^[2]

Compounds that contain hydrogen attached to nitrogen, such as ammonia or primary or secondary amines, will rapidly react with MIC to form substituted ureas. Other N-H compounds, such as amides and ureas, react much more slowly with MIC.^[9]

Alcohols and phenols, which contain an O-H group, react slowly with MIC, but the reaction can be catalyzed by trialkylamines or dialkyltin dicarboxylate.

Oximes, hydroxylamines, and enols also react with MIC to form methylcarbamates.^[2] These reactions produce the products described below (Uses).

When treated with catalysts, MIC reacts with itself to form a solid trimer, trimethyl isocyanurate, or a higher molecular weight polymer.

Sodium methoxide, triethyl phosphine, ferric chloride, and certain other metal compounds catalyze the formation of the MIC-trimer, while the higher molecular weight polymer formation is catalyzed by certain trialkylamines. Since the formation of the MIC trimer is exothermic (298 calories per gram of MIC), the reaction can lead to violent boiling of the MIC. The high-molecular-weight-polymer hydrolyzes in hot water to form the trimethyl isocyanurate. Since catalytic metal salts can be formed from impurities in commercial grade MIC and steel, this product must not be stored in steel drums or tanks.^[2]

Uses

Methyl isocyanate is an intermediate chemical in the production of carbamate pesticides (such as carbaryl, carbofuran, methomyl, and aldicarb). It has also been used in the production of rubbers and adhesives.

Hazards

Methyl isocyanate (MIC) is extremely toxic. The threshold limit value set by the American Conference on Government Industrial Hygienist was 0.02 ppm which is an extremely low concentration. MIC can damage by inhalation, ingestion and contact in quantities as low as 0.4 ppm. Damage includes coughing, chest pain, dyspnea, asthma, irritation of the eyes, nose and throat as well as skin damage. Higher levels of exposure, over 21 ppm, can result in pulmonary or lung edema, emphysema and hemorrhages, bronchial pneumonia and death. Although the detectable odor of methyl isocyanate is a concentration triple the permissible exposure, its potent lachrymal properties provide an excellent warning of its presence (at a concentration of 2-4 parts per million (ppm) subject's eyes are irritated, while at 21 ppm subjects could not tolerate the presence of methyl isocyanate in air).^[10]

Proper care must be taken to store methyl isocyanate because of its ease of exothermically polymerizing (see Reactions) and its similar sensitivity to water. Only stainless steel or glass containers may be safely used; the MIC must be stored at temperatures below 40 °C.

The toxic effect of the compound was apparent in the Bhopal disaster, when around 43,000 kilograms of methyl isocyanate and other gases were released over a populated area on December 3, 1984, killing 3,828 people as well as leaving thousands more with varying disabilities.

References

• Ingrid Eckerman (2005). The Bhopal Saga: Causes and Consequences of the Worlds Largest Industrial Disaster. Universities Press India. ISBN 81-7371-515-7.  [1]

1. ^ Incident Response and Settlement. Bhopal Information Center.
2. ^ ^{a b c d} Union Carbide Corporation "Methyl Isocyanate” Product Information Publication, F-41443, November 1967.
3. ^ Slocombe R. J. & Hardy E. E., “Process of Producing Carbamyl Chlorides” U. S. Patent No. 2,480,088, August 23, 1949.
4. ^ Merz W, “Procédé et dispositif de préparation d’isocyanates d’alkyle” French Patent No. 1,400,863 assigned to Farbenfabriken Bayer AG, Germany 1965.
5. ^ Chemical Week, “A fleeting existence for toxic-gas molecules” p. 9, June 12, 1985.
6. ^ Giesselmann G., Guenther K., Fuenten W., “Methyl Isocyanate”, German Patent No. 2,828,259, January 10, 1980 (cited in Chemical Abstracts 92: 214882n).
7. ^ Chemical Week, “A safer method for making carbamates” p. 136, no. 20, 1985b.
8. ^ Enrique A. Castro, Roy B. Moodie and Peter J. Sansom (1985). "The kinetics of hydrolysis of methyl and phenyl lsocyanates". J. Chem. Soc., Perkin Trans. 2 1985: 737. doi:10.1039/P29850000737.
9. ^ March J. (1985). Advanced Organic Chemistry, 3rd ed., New York: John Wiley & Sons, 802. 
10. ^ Kimmerle G, and Eben A., “Zur Toxicität von Methylisocyanat und dessen quantitativer Bestimmung in der Luft”, Achiv fur Toxikologie, no. 20, 235-241, 1964.