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| Names | |
|---|---|
| Preferred IUPAC name Isocyanatomethane | |
| Other names Methyl carbylamine MIC | |
| Identifiers | |
| |
3D model (JSmol) | |
| ChEBI | |
| ChemSpider |
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| ECHA InfoCard | 100.009.879 |
| UNII | |
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| 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 | 38.3–41 °C (100.9–105.8 °F; 311.4–314.1 K)[2] |
| 10% (15°C)[1] | |
| Vapor pressure | 57.7kPa |
| Structure | |
| 2.8D | |
| Thermochemistry | |
Std enthalpy of formation(ΔfH⦵298) | −92.0kJ·mol−1[2] |
Std enthalpy of combustion(ΔcH⦵298) | -1.1275E+06J/mol[3] |
| Hazards | |
| GHS labelling: | |
     | |
| 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) | |
| 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): | |
LD50 (median dose) | 120 mg/kg (oral, mouse) 51.5 mg/kg (oral, rat)[4] |
LC50 (median concentration) | 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). | |
Methyl isocyanate (MIC) is anorganic compound with the molecular formula CH3NCO. Synonyms areisocyanatomethane andmethyl carbylamine. Methyl isocyanate is an intermediate chemical in the production ofcarbamatepesticides and Haffmann Bromamide Degradation (such ascarbaryl,carbofuran,methomyl, andaldicarb). It has also been used in the production ofrubbers andadhesives. As an extremelytoxic and 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 approximately 15,000-20,000.[6][7][8][9][10] It is also a very potentlachrymatory agent.[11]
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 (seeReactions below).
It has a refractive index of 1.363 with a wavelength of 589 nm at a temperature of 20 °C.[13]
Methyl isocyanate is usually manufactured by the reaction ofmonomethylamine andphosgene. 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 chloride is formed, but N-methylcarbamoyl chloride (MCC) forms as the mixture is condensed, leaving onemole ofhydrogen chloride as 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 usingdistillation techniques.[15]
Methyl isocyanate is also manufactured fromN-methylformamide and 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]
Methyl isocyanate reacts readily with many substances that contain N-H or O-H groups. With water, it forms1,3-dimethylurea andcarbon dioxide with 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 disaster after a large amount of water was introduced to a MIC storage tank. The consequence of the out of controlexothermic process was arunaway reaction and the direct release of 42 tons of MIC to the atmosphere.
If MIC is in excess,1,3,5-trimethylbiuret is formed along withcarbon dioxide.[12]Alcohols andphenols, which contain an O-H group, react slowly with MIC, but the reaction can be catalyzed by trialkylamines or dialkyltin dicarboxylate.Oximes,hydroxylamines, andenols also react with MIC to form methylcarbamates.[12] These reactions produce the products described below (Uses).
Ammonia, primary, and secondaryamines rapidly 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 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 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. Sincecatalytic 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]
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,chest pain,dyspnea,asthma, irritation of theeyes,nose andthroat, as well asskin damage. Higher levels of exposure, over 21 ppm, can result in pulmonary or lungedema,emphysema andhemorrhages, bronchialpneumonia anddeath. Although the odor of methyl isocyanate cannot be detected at 5 ppm by most people, its potentlachrymal properties 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] The irritant effects of methyl isocyanate are mediated by the irritant and tear gas receptorTRPA1 in pain-sensing nerve endings in the eye.[22]
Proper care must be taken to store methyl isocyanate because of its ease of exothermically polymerizing (seeReactions) and its similar sensitivity to water. Onlystainless steel orglass 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 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.[23][24]
During structural fires, natural materials can contribute to releasing isocyanates including methyl isocyanate.[25]
Until recent decades, the mechanism of methyl isocyanate toxicity in humans was largely unknown or unclear.[26][27] Methyl isocyanate and otherisocyanates areelectrophiles and are currently thought to cause toxicity by thealkylation of biomolecules.[28] The mechanism of methyl isocyanate was previously suspected to be thecarbamylation ofhemoglobin, thus interfering with its oxygen-binding capability and causinghypoxia. However, experiments showed that whenrats andguinea pigs were exposed to methyl isocyanate atconcentrations above 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.[29][30]
On 30 July 2015, scientists reported that upon the first touchdown of thePhilae lander oncomet67/P's surface, measurements by theCOSAC and Ptolemy instruments revealed sixteenorganic compounds, four of which were seen for the first time on acomet, includingacetamide,acetone, methyl isocyanate andpropionaldehyde.[32][33][34]
In 2017, two teams of astronomers using theAtacama Large Millimeter Array (ALMA)interferometer made of 66 radio telescopes in theAtacama Desert (northernChile) have discovered the presence of MIC around young Sun-like stars.[31]
MIC is considered aprebiotic molecule as 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 ofpeptides andamino acids, which, in the form ofproteins, are the biological basis for life as we know it".[31]
| International | |
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| Other | |
