Methyl isocyanate

Overview
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.

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. Methyl isocyanate is soluble in water to 6-10 parts per 100 parts, but it reacts with the water (see Reactions).

Manufacture
Methyl isocyanate is usually manufactured from monomethylamine and phosgene. These substances react at a range of temperatures, but 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 and leaves one mole of hydrogen chloride as a gas.

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



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. Other manufacturing methods have been reported.

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; 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.

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.

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. 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.

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).

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 40ºC.

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.