Methyl isocyanate

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Methyl isocyanate
Methyl isocyanate
Methyl isocyanate
Identifiers
CAS number 624-83-9 YesY
PubChem 12228
ChemSpider 11727 YesY
ChEBI CHEBI:59059 YesY
Jmol-3D images Image 1
Properties
Molecular formula C2H3NO
Molar mass 57.051 g/mol
Appearance Colorless liquid
Odor Sharp, pungent odor[1]
Density 0.9230 g/cm3 at 27°C
Melting point −45°C
Boiling point 39.5°C
Solubility in water very soluble[2]
Vapor pressure 57.7 kPa
Structure
Dipole moment 2.8 D
Thermochemistry
Std enthalpy of
formation
ΔfHo298
−92.0 kJ·mol−1[2]
Hazards
NFPA 704
Flammability code 3: Liquids and solids that can be ignited under almost all ambient temperature conditions. Flash point between 23 and 38 °C (73 and 100 °F). E.g., gasoline) Health code 4: Very short exposure could cause death or major residual injury. E.g., VX gas Reactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g., phosphorus Special hazard W: Reacts with water in an unusual or dangerous manner. E.g., cesium, sodiumNFPA 704 four-colored diamond
Flash point −7 °C (19 °F; 266 K)
Explosive limits 5.3–26%[2]
Related compounds
Related compounds Methyl isothiocyanate
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 YesY (verify) (what is: YesY/N?)
Infobox references

Methyl isocyanate (MIC) is an organic compound with the molecular formula CH3NCO. Synonyms are isocyanatomethane, methyl carbylamine, and MIC. 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 extremely hazardous to human health. It was the principal toxicant involved in the Bhopal disaster, which killed nearly 8,000 people initially and approximately 20,000 to 30,000 people in total.[3][4][5][6][7][8][9]

Physical properties[edit]

Methyl isocyanate (MIC) is a colorless, lachrymatory (tearing agent), flammable liquid.[10] It is soluble in water to 6–10 parts per 100 parts, but it also reacts with water (see Reactions below).

Manufacture[edit]

Methyl isocyanate is usually manufactured by the reaction of monomethylamine and phosgene. For large scale production, it is advantageous to combine these reactants at higher temperature in the gas phase. A mixture of methyl isocyanate and two moles of hydrogen chloride is formed, but N-methylcarbamoyl chloride (MCC) forms as the mixture is condensed, leaving one mole of hydrogen chloride as a gas.

MMA plus Phosgene diagram.png

The methyl isocyanate is obtained by treating the MCC with a tertiary amine, such as N,N-dimethylaniline, or with pyridine[11] or by separating it by using distillation techniques.[12]

MCC to MIC & HCl.png

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.[13] Other manufacturing methods have been reported.[14][15]

Reactions[edit]

Methyl isocyanate reacts readily with many substances that contain N-H or O-H groups. With water, it forms 1,3-dimethylurea and carbon dioxide with the evolution of heat (325 calories per gram of MIC):

MIC & water to DMU & TMB.png

At 25 °C, in excess water, half of the MIC is consumed in 9 min.;[16] 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.[10] 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.[10] These reactions produce the products described below (Uses).

MIC plus 1-naphthol to carbaryl.png

Ammonia, primary, and secondary amines rapidly react with MIC to form substituted ureas. Other N-H compounds, such as amides and ureas, react much more slowly with MIC.[17]

It also reacts with itself to form a trimer or higher molecular weight polymers. In the presence of catalysts, MIC reacts with itself to form a solid trimer, trimethyl isocyanurate, or a higher molecular weight polymer:

MIC to trimer.svg

Sodium methoxide, triethyl phosphine, ferric chloride and certain other metal compounds catalyze the formation of the MIC-trimer, while the high-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.[10]

Uses[edit]

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[edit]

Methyl isocyanate (MIC) is extremely toxic. The threshold limit value set by the American Conference on Government Industrial Hygienists is 0.02 ppm. MIC is toxic by inhalation, ingestion and contact in quantities as low as 0.4 ppm. Exposure symptoms 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 odor of methyl isocyanate cannot be detected at 5 ppm by most people, 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).[18]

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 (104 °F) and preferably at 4 °C (39 °F).

The toxic effect of the compound was apparent in the Bhopal disaster, when around 42,000 kilograms (93,000 lb) of methyl isocyanate and other gases were released from the underground reservoirs of the Union Carbide India Limited (UCIL) factory, over a populated area on December 3, 1984, immediately killing thousands and leading to the deaths of tens of thousands in subsequent weeks and months.

References[edit]

  1. ^ CDC - NIOSH Pocket Guide to Chemical Hazards
  2. ^ a b c Lide, David R., ed. (2006). CRC Handbook of Chemistry and Physics (87th ed.). Boca Raton, FL: CRC Press. ISBN 0-8493-0487-3. 
  3. ^ Methyl Isocyanate. Union Carbide F-41443A – 7/76. Union Carbide Corporation, New York 1976
  4. ^ Operating Manual Part II. Methyl Isocyanate Unit. Union Carbide India Limited, Agricultural Products Division, 1979
  5. ^ Broughton, E. (2005). "The Bhopal disaster and its aftermath: a review". Environmental Health 4 (1): 6. doi:10.1186/1476-069X-4-6. PMC 1142333. PMID 15882472. 
  6. ^ Eckerman, I. (2001). "Chemical Industry and Public Health — Bhopal as an example" (pdf). MPH (Göteborg, Sweden: Nordic School of Public Health) 2001 (24). ISSN 1104-5701. 
  7. ^ Eckerman, I. (2004). The Bhopal Saga - Causes and Consequences of the World's Largest Industrial Disaster. India: Universities Press. ISBN 81-7371-515-7. 
  8. ^ Rosenberg, J. "At 1984 - Huge Poison Gas Leak in Bhopal, India". About.com. Retrieved 2008-07-10. 
  9. ^ Eckerman, Ingrid (2013). Bhopal Catastrophe 1984: Causes and Consequences (in Reference Module in Earth Systems and Environmental Sciences). Elsevier. 
  10. ^ a b c d Union Carbide Corporation "Methyl Isocyanate" Product Information Publication, F-41443, November 1967.
  11. ^ US patent 2480088, Slocombe, R. J.; Hardy, E. E., "Process of Producing Carbamyl Chlorides", issued 1949-08-23, assigned to Monsanto 
  12. ^ FR patent 1400863, Merz, W., "Procédé et dispositif de préparation d'isocyanates d'alkyle", issued 1965-05-28, assigned to Bayer 
  13. ^ Chemical Week, "A fleeting existence for toxic-gas molecules" p. 9, June 12, 1985.
  14. ^ DE patent 2828259, Giesselmann, G.; Guenther, K.; Fuenten, W., "Verfahren zur Herstellung von Methyl Isocyanate", issued 1980-01-10, assigned to Degussa 
  15. ^ "A safer method for making carbamates". Chemical Week. 1985b (20): 136. 1985. 
  16. ^ Castro, E. A.; Moodie, R. B.; Sansom, P. J. (1985). "The kinetics of hydrolysis of methyl and phenyl isocyanates". Journal of the Chemical Society, Perkin Transactions 2 1985 (5): 737–742. doi:10.1039/P29850000737. 
  17. ^ March, J. (1985). Advanced Organic Chemistry (3rd ed.). New York: John Wiley & Sons. p. 802. 
  18. ^ Kimmerle, G.; Eben, A. (1964). "Zur Toxizität von Methylisocyanat und dessen quantitativer Bestimmung in der Luft". Archiv für Toxikologie 20 (4): 235–241. 

External links[edit]