Wikipedia

Methyl isocyanate

Not to be confused withMethyl isocyanide.

Methyl isocyanate(MIC) is anorganic compoundwith the molecular formula CH3NCO. Synonyms areisocyanatomethaneandmethyl carbylamine.Methyl isocyanate is an intermediate chemical in the production ofcarbamatepesticides(such ascarbaryl,carbofuran,methomyl,andaldicarb). It has also been used in the production ofrubbersandadhesives.As an extremelytoxicand irritating compound, it is very hazardous to human health. MIC was the principal toxicant involved in theBhopal gas disaster,which short-term killed 4,000–8,000 people and caused permanent injury and premature deaths to tens of thousands more.[6][7][8][9][10]It is also a very potentlachrymatory agent.[11]

Methyl isocyanate
Names
Preferred IUPAC name
Isocyanatomethane
Other names
Methyl carbylamine
MIC
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.009.879Edit this at Wikidata
UNII
  • InChI=1S/C2H3NO/c1-3-2-4/h1H3checkY
    Key: HAMGRBXTJNITHG-UHFFFAOYSA-NcheckY
  • O=C=NC
Properties
C2H3NO
Molar mass 57.051 g/mol
Appearance Colorless liquid
Odor Sharp, pungent odor[1]
Density 0.9230 g/cm3at 27 °C
Melting point −45 °C (−49 °F; 228 K)[2]
Boiling point 38.3–41 °C (100.9–105.8 °F; 311.4–314.1 K)[2]
10% (15°C)[1]
Vapor pressure 57.7kPa
Structure
2.8D
Thermochemistry
−92.0kJ·mol−1[2]
-1.1275E+06J/mol[3]
Hazards
GHSlabelling:
GHS06: ToxicGHS08: Health hazardGHS02: FlammableGHS05: CorrosiveGHS07: Exclamation mark
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+P310,P302+P352,P303+P361+P353,P304+P340,P304+P341,P305+P351+P338,P308+P313,P310,P312,P320,P321,P322,P330,P332+P313,P333+P313,P342+P311,P361,P362,P363,P370+P378,P403+P233,P403+P235,P405,P501
NFPA 704(fire diamond)
NFPA 704 four-colored diamondHealth 4: Very short exposure could cause death or major residual injury. E.g. VX gasFlammability 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. gasolineInstability 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 acid
4
3
3
W
Flash point −7 °C (19 °F; 266 K)
534 °C (993 °F; 807 K)
Explosive limits 5.3–26%[2]
Lethal doseor concentration (LD, LC):
120 mg/kg (oral, mouse)
51.5 mg/kg (oral, rat)[4]
6.1 ppm (rat, 6 hr)
12.2 ppm (mouse, 6 hr)
5.4 ppm (guinea pig, 6 hr)
21 ppm (rat, 2 hr)[4]
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 theirstandard state(at 25 °C [77 °F], 100 kPa).

Physical properties

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Methyl isocyanate is a colorless, poisonous,lachrymatory(tearingagent), flammable liquid.[12]It is soluble in water to 6–10 parts per 100 parts, but it also reacts with water (seeReactionsbelow).

It has a refractive index of 1.363 with a wavelength of 589 nm at a temperature of 20 °C[13]

Manufacture

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Methyl isocyanate is usually manufactured by the reaction ofmonomethylamineandphosgene.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 ofhydrogen chlorideis formed, but N-methylcarbamoyl chloride (MCC) forms as the mixture is condensed, leaving onemoleofhydrogen chlorideas a gas.

The methyl isocyanate is obtained by treating the MCC with atertiary amine,such asN,N-dimethylaniline,or withpyridine,[14]or by separating it by usingdistillationtechniques.[15]

Methyl isocyanate is also manufactured fromN-methylformamideand air. In the latter process, it is immediately consumed in a closed-loop process to makemethomyl.[16]Other manufacturing methods have been reported.[17][18]

Reactions

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Methyl isocyanate reacts readily with many substances that contain N-H or O-H groups. With water, it forms1,3-dimethylureaandcarbon dioxidewith the evolution of heat (1358.5 joules, or 325 calories, per gram of MIC): It is relatively slow to react at below 68 °F, but will increase its rate with elevated temperatures or in the presence of acid or base.[19]

At 25 °C, in excess water, half of the MIC is consumed in 9 min.;[20]if the heat is not efficiently removed from the reacting mixture, the rate of the reaction will increase and rapidly cause the MIC to boil. Such a reaction triggered theBhopal disasterafter a large amount of water was introduced to a MIC storage tank. The consequence of the out of controlexothermic processwas arunaway reactionand the direct release of 42 tons of MIC to the atmosphere.

If MIC is in excess,1,3,5-trimethylbiuretis formed along withcarbon dioxide.[12]Alcoholsandphenols,which contain an O-H group, react slowly with MIC, but the reaction can be catalyzed by trialkylaminesor dialkyltin dicarboxylate.Oximes,hydroxylamines,andenolsalso react with MIC to form methylcarbamates.[12]These reactions produce the products described below (Uses).

Ammonia,primary, and secondaryaminesrapidly react with MIC to form substitutedureas.Other N-H compounds, such as amides andureas,react much more slowly with MIC.[21]

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

Sodium methoxide,triethyl phosphine,ferric chlorideand 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 isexothermic(1246 joules, or 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 trimethylisocyanurate.Sincecatalyticmetal salts can be formed from impurities in commercial grade MIC and steel, this product must not be stored in steel drums or tanks.[12]

Toxicity

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Methyl isocyanate is extremelytoxic.There is no knownantidote.The threshold limit value set by the American Conference of Governmental Industrial Hygienists is 0.02ppm.MIC is toxic by inhalation, ingestion and contact in quantities as low as 0.4ppm.Exposure symptoms include coughing,chestpain,dyspnea,asthma,irritation of theeyes,noseandthroat,as well asskindamage. Higher levels of exposure, over 21 ppm, can result in pulmonary or lungedema,emphysemaandhemorrhages,bronchialpneumoniaanddeath.Although the odor of methyl isocyanate cannot be detected at 5 ppm by most people, its potentlachrymalproperties provide an excellent warning of its presence (at a concentration of 2–4 parts per million (ppm) subjected to eyes are irritated, while at 21 ppm, subjects could not tolerate the presence of methyl isocyanate in air).[11]

Proper care must be taken to store methyl isocyanate because of its ease of exothermically polymerizing (seeReactions) and its similar sensitivity to water. Onlystainless steelorglasscontainers 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 1984Bhopal disaster,when around 42,000 kilograms (93,000 lb) of methyl isocyanate and other gases were released from the underground reservoirs of theUnion Carbide India Limited(UCIL) factory, over a populated area on 3 December 1984, killing about 3,500 people immediately, 8,000 people in the first 48 hours and 15,000 more over the next several years. 200,000 people had lasting health effects from the disaster.[22][23]

During structural fires, natural materials can contribute to releasing isocyanates including methyl isocyanate.[24]

Mechanism of action

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Until recent decades, the mechanism of methyl isocyanate toxicity in humans was largely unknown or unclear.[25][26]Methyl isocyanate and otherisocyanatesareelectrophilesand are currently thought to cause toxicity by thealkylationof biomolecules.[27]The mechanism of methyl isocyanate was previously suspected to be thecarbamylationofhemoglobin,thus interfering with its oxygen-binding capability and causinghypoxia.However, experiments showed that whenratsandguinea pigswere exposed to methyl isocyanate atconcentrationsabove themedian lethal concentration(LC50,the concentration sufficient to kill 50% of the tested population), only 2% of hemoglobin molecules were carbamylated, suggesting that this is probably not the mechanism of toxicity.[28][29]

Extraterrestrial occurrence

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A photographic montage of methyl isocyanate around young Sun-like stars, as detected by theALMAinterferometer(northernChile).[30]

On 30 July 2015, scientists reported that upon the first touchdown of thePhilaelander oncomet67/P'ssurface, measurements by theCOSAC and Ptolemy instrumentsrevealed sixteenorganic compounds,four of which were seen for the first time on acomet,includingacetamide,acetone,methyl isocyanate andpropionaldehyde.[31][32][33]

In 2017, two teams of astronomers using theAtacama Large Millimeter Array(ALMA)interferometermade of 66 radio telescopes in theAtacama Desert(northernChile) have discovered the presence of MIC around young Sun-like stars.[30]

MIC is considered aprebiotic moleculeas explained by the discoverers of the ALMA findings inIRAS 16293-2422,a multiple system of very young stars: "This family of organic molecules is involved in the synthesis ofpeptidesandamino acids,which, in the form ofproteins,are the biological basis for life as we know it ".[30]

References

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  2. ^abcdLide, David R., ed. (2006).CRC Handbook of Chemistry and Physics(87th ed.). Boca Raton, FL:CRC Press.ISBN0-8493-0487-3.
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  4. ^ab"Methyl isocyanate".Immediately Dangerous to Life or Health Concentrations (IDLH).National Institute for Occupational Safety and Health(NIOSH).
  5. ^"NFPA Hazard Rating Information for Common Chemicals".nmsu.edu. Archived fromthe originalon 17 February 2015.Retrieved10 June2021.
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  7. ^Eckerman I (2001)."Chemical Industry and Public Health — Bhopal as an example"(PDF).MPH.2001(24). Göteborg, Sweden: Nordic School of Public Health.ISSN1104-5701.Archived(PDF)from the original on 30 October 2012.
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  11. ^abKimmerle 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.S2CID21422558.
  12. ^abcdUnion Carbide Corporation "Methyl Isocyanate" Product Information Publication, F-41443, November 1967.
  13. ^Kirilin, Aleksei; Belova, Liya; Pletneva, Maria (January–February 2017). "New aspects of isocyanate synthesis with the use of O-silylurethanes".Mendeleev Communications.27(1): 99–100.doi:10.1016/j.mencom.2017.01.033.
  14. ^US patent 2480088,Slocombe, R. J.; Hardy, E. E., "Process of Producing Carbamyl Chlorides", issued 1949-08-23, assigned to Monsanto
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  18. ^"A safer method for making carbamates".Chemical Week.1985b (20): 136. 1985.
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