Methyl isocyanate

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Methyl isocyanate
Methyl isocyanate
Methyl isocyanate
Names
Preferred IUPAC name
Isocyanatomethane
Other names
Methyl carbylamine
MIC
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.009.879
Properties
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 (−49 °F; 228 K) [2]
Boiling point 38.3 °C (100.9 °F; 311.4 K) [2]
10% (15°C)[1]
Vapor pressure 57.7 kPa
Structure
2.8 D
Thermochemistry
−92.0 kJ·mol−1[2]
Hazards
GHS pictograms GHS06: Toxic GHS08: Health hazard GHS02: Flammable GHS05: Corrosive GHS07: Harmful
H225, H300, H311, H315, H317, H318, H330, H334, H335, H361d
P201, P202, P210, P233, P240, P241, P242, P243, P260, P261, P264, P270, P271, P272, P280, P281, P284, P285, P301+310, P302+352, P303+361+353, P304+340, P304+341, P305+351+338, P308+313
NFPA 704 (fire diamond)
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. gasolineHealth code 4: Very short exposure could cause death or major residual injury. E.g. VX gasReactivity code 3: Capable of detonation or explosive decomposition but requires a strong initiating source, must be heated under confinement before initiation, reacts explosively with water, or will detonate if severely shocked. E.g. hydrogen peroxideSpecial hazard W: Reacts with water in an unusual or dangerous manner. E.g. sodium, sulfuric acidNFPA 704 four-colored diamond
3
4
3
Flash point −7 °C (19 °F; 266 K)
534 °C (993 °F; 807 K)
Explosive limits 5.3–26%[2]
Lethal dose or concentration (LD, LC):
120 mg/kg (oral, mouse)
51.5 mg/kg (oral, rat)[3]
6.1 ppm (rat, 6 hr)
12.2 ppm (mouse, 6 hr)
5.4 ppm (guinea pig, 6 hr)
21 ppm (rat, 2 hr)[3]
NIOSH (US health exposure limits):
PEL (Permissible)
TWA 0.02 ppm (0.05 mg/m3) [skin][1]
REL (Recommended)
TWA 0.02 ppm (0.05 mg/m3) [skin][1]
IDLH (Immediate danger)
3 ppm[1]
Related compounds
Related compounds
Methyl isothiocyanate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☑Y verify (what is ☑Y☒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 3,787 people initially and officially 19,787 people in total.[5][6][7][8][9][10][11]

Physical properties[edit]

Methyl isocyanate is a colorless, poisonous, lachrymatory (tearing agent), flammable liquid.[12] 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.svg

The methyl isocyanate is obtained by treating the MCC with a tertiary amine, such as N,N-dimethylaniline, or with pyridine[13] or by separating it by using distillation techniques.[14]

MCC to MIC & HCl.svg

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.[15] Other manufacturing methods have been reported.[16][17]

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.;[18] 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.[12] 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.[12] These reactions produce the products described below (Uses).

MIC plus 1-naphthol to carbaryl.svg

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.[19]

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.[12]

Hazards[edit]

Methyl isocyanate 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 include 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).[20]

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).[citation needed]

The toxic effect of the compound was apparent in the 1984 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, killing about 3,500 people immediately and 15,000 more over the next several years.[21]

Mechanism of Action[edit]

Until recent decades, the mechanism of methyl isocyanate toxicity in humans was largely unknown or unclear. [22][23] Methyl isocyanate and other isocyanates are electrophiles and are currently thought to cause toxicity by the alkylation of biomolecules. [24] The mechanism of methyl isocyanate was previous suspected to be the carbamylation of hemoglobin which would interfere with its oxygen binding capability causing hypoxia. However, experiments showed that when rats and guinea pigs were exposed to methyl isocyanate at concentrations above the LC50, only 2% of hemoglobin molecules were carbamylated suggesting that this is likely not the mechanism of toxicity. [25][26]

Extraterrestrial occurrence[edit]

Methyl isocyanate around young Sun-like stars detected by ALMA.[27]

On 30 July 2015, scientists reported that upon the first touchdown of the Philae lander on comet 67/P's surface, measurements by the COSAC and Ptolemy instruments revealed sixteen organic compounds, four of which were seen for the first time on a comet, including acetamide, acetone, methyl isocyanate and propionaldehyde.[28][29][30]

References[edit]

  1. ^ a b c d e NIOSH Pocket Guide to Chemical Hazards. "#0423". National Institute for Occupational Safety and Health (NIOSH).
  2. ^ a b c d Lide, David R., ed. (2006). CRC Handbook of Chemistry and Physics (87th ed.). Boca Raton, FL: CRC Press. ISBN 0-8493-0487-3.
  3. ^ a b "Methyl isocyanate". Immediately Dangerous to Life and Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
  4. ^ "Archived copy". Archived from the original on 17 February 2015. Retrieved 15 March 2015.CS1 maint: archived copy as title (link)
  5. ^ Methyl Isocyanate. Union Carbide F-41443A – 7/76. Union Carbide Corporation, New York 1976
  6. ^ Operating Manual Part II. Methyl Isocyanate Unit. Union Carbide India Limited, Agricultural Products Division, 1979
  7. ^ Broughton E (May 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.
  8. ^ 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. Archived (PDF) from the original on 30 October 2012.
  9. ^ Eckerman I (2004). The Bhopal Saga - Causes and Consequences of the World's Largest Industrial Disaster. India: Universities Press. ISBN 81-7371-515-7. Archived from the original on 10 June 2007.
  10. ^ Rosenberg J. "At 1984 - Huge Poison Gas Leak in Bhopal, India". About.com. Archived from the original on 2 December 2007. Retrieved 10 July 2008.
  11. ^ Eckerman I (2013). "Bhopal Gas Catastrophe 1984: Causes and Consequences". Reference Module in Earth Systems and Environmental Sciences. Elsevier. pp. 272–287. doi:10.1016/B978-0-12-409548-9.01903-5. ISBN 978-0-12-409548-9.
  12. ^ a b c d Union Carbide Corporation "Methyl Isocyanate" Product Information Publication, F-41443, November 1967.
  13. ^ US patent 2480088, Slocombe, R. J.; Hardy, E. E., "Process of Producing Carbamyl Chlorides", issued 1949-08-23, assigned to Monsanto 
  14. ^ FR patent 1400863, Merz, W., "Procédé et dispositif de préparation d'isocyanates d'alkyle", issued 1965-05-28, assigned to Bayer 
  15. ^ Chemical Week, "A fleeting existence for toxic-gas molecules" p. 9, June 12, 1985.
  16. ^ DE patent 2828259, Giesselmann, G.; Guenther, K.; Fuenten, W., "Verfahren zur Herstellung von Methyl Isocyanate", issued 1980-01-10, assigned to Degussa 
  17. ^ "A safer method for making carbamates". Chemical Week. 1985b (20): 136. 1985.
  18. ^ Castro EA, Moodie RB, Sansom PJ (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.
  19. ^ March J (1985). Advanced Organic Chemistry (3rd ed.). New York: John Wiley & Sons. p. 802.
  20. ^ Kimmerle G, Eben A (1964). "Zur Toxizität von Methylisocyanat und dessen quantitativer Bestimmung in der Luft". Archiv für Toxikologie. 20 (4): 235–241. doi:10.1007/bf00577897.
  21. ^ "Bhopal trial: Eight convicted over India gas disaster". BBC News. 7 June 2010. Archived from the original on 7 June 2010. Retrieved 7 June 2010.
  22. ^ Mehta PS, Mehta AS, Mehta SJ, Makhijani AB (December 1990). "Bhopal tragedy's health effects. A review of methyl isocyanate toxicity". JAMA. 264 (21): 2781–7. doi:10.1001/jama.1990.03450210081037. PMID 2232065.
  23. ^ Varma DR (June 1987). "Epidemiological and experimental studies on the effects of methyl isocyanate on the course of pregnancy". Environmental Health Perspectives. 72: 153–7. doi:10.1289/ehp.8772153. PMC 1474644. PMID 3622430.
  24. ^ Bessac, B. F.; Jordt, S.-E. (1 July 2010). "Sensory Detection and Responses to Toxic Gases: Mechanisms, Health Effects, and Countermeasures". Proceedings of the American Thoracic Society. 7 (4): 269–277. doi:10.1513/pats.201001-004sm. ISSN 1546-3222. PMC 3136963. PMID 20601631.
  25. ^ Varma, Daya R.; Guest, Ian (1993). "The Bhopal accident and methyl isocyanate toxicity". Journal of Toxicology and Environmental Health. 40 (4): 513–529. doi:10.1080/15287399309531816. ISSN 0098-4108. PMID 8277516.
  26. ^ Ramachandran, P.K.; Gandhe, B.R.; Venkateswaran, K.S.; Kaushik, M.P.; Vijayaraghavan, R.; Agarwal, G.S.; Gopalan, N.; Suryanarayana, M.V.S.; Shinde, S.K.; Sriramachari, S. (1988). "Gas chromatographic studies of the carbamylation of haemoglobin by methyl isocyanate in rats and rabbits". Journal of Chromatography B: Biomedical Sciences and Applications. 426 (2): 239–247. doi:10.1016/s0378-4347(00)81952-0. ISSN 0378-4347. PMID 3392138.
  27. ^ "ALMA Finds Ingredient of Life Around Infant Sun-like Stars". www.eso.org. Archived from the original on 8 June 2017. Retrieved 8 June 2017.
  28. ^ Jordans F (30 July 2015). "Philae probe finds evidence that comets can be cosmic labs". The Washington Post. Associated Press. Retrieved 30 July 2015.
  29. ^ "Science on the Surface of a Comet". European Space Agency. 30 July 2015. Archived from the original on 2 August 2015. Retrieved 30 July 2015.
  30. ^ Bibring JP, Taylor MG, Alexander C, Auster U, Biele J, Finzi AE, et al. (July 2015). "Philae's first look. Philae's First Days on the Comet. Introduction". Science. 349 (6247): 493. Bibcode:2015Sci...349..493B. doi:10.1126/science.aac5116. PMID 26228139.

External links[edit]