Chloromethane

Occurrence

Chloromethane is the most abundant organohalogen, anthropogenic or natural, in the atmosphere.

Marine

Laboratory cultures of marine phytoplankton (Phaeodactylum tricornutum, Phaeocystis sp., Thalassiosira weissflogii, Chaetoceros calcitrans, Isochrysis sp., Porphyridium sp., Synechococcus sp., Tetraselmis sp., Prorocentrum sp., and Emiliana huxleyi) produce CH₃Cl, but in relatively insignificant amounts. An extensive study of 30 species of polar macroalgae revealed the release of significant amounts of CH₃Cl in only Gigartina skottsbergii and Gymnogongrus antarcticus.

Biogenesis

The salt marsh plant Batis maritima contains the enzyme methyl chloride transferase that catalyzes the synthesis of CH₃Cl from S-adenosine-L-methionine and chloride. This protein has been purified and expressed in E. coli, and seems to be present in other organisms such as white rot fungi (Phellinus pomaceus), red algae (Endocladia muricata), and the ice plant (Mesembryanthemum crystallinum), each of which is a known CH₃Cl producer.

Production

Large amounts of chloromethane are produced naturally in the oceans by the action of sunlight on biomass and chlorine in sea foam. However, all chloromethane that is used in industry is produced synthetically.

Most chloromethane is prepared by reacting methanol with hydrogen chloride, according to the chemical equation

This can be carried out either by bubbling hydrogen chloride gas through boiling methanol with or without a zinc chloride catalyst, or by passing combined methanol and hydrogen chloride vapors over an alumina catalyst at 350 °C (662 °F).

A smaller amount of chloromethane is produced by heating a mixture of methane and chlorine to over 400 °C (752 °F). However, this method also results in more highly chlorinated compounds such as dichloromethane, chloroform, and carbon tetrachloride and is usually only used when these other products are also desired.

Uses

Chloromethane was a widely used refrigerant, but its use has been discontinued due to its toxicity and flammability. Chloromethane was also once used for producing lead-based gasoline additives (tetramethyllead).

The most important use of chloromethane today is as a chemical intermediate in the production of silicone polymers. Smaller quantities are used as a solvent in the manufacture of butyl rubber and in petroleum refining.

Chloromethane is employed as a methylating and chlorinating agent in organic chemistry. It is also used in a variety of other fields: as an extractant for greases oils and resins, as a propellant and blowing agent in polystyrene foam production, as a local anesthetic, as an intermediate in drug manufacturing, as a catalyst carrier in low-temperature polymerization, as a fluid for thermometric and thermostatic equipment, and as a herbicide.

Safety

Inhalation of chloromethane gas produces central nervous system effects similar to drug intoxication. Exposure may cause drowsiness, dizziness, or confusion and difficulty breathing, walking or speaking may occur. At higher concentrations, paralysis, seizures, and coma can result.

In case of ingestion, nausea and vomiting may occur. Skin contact, when in the form of a refrigerated liquid, may result in frostbite. Contact with the eyes may result in dim vision, and widely dilated pupils that react slowly to changes in light.

Chronic exposure to chloromethane has been linked to birth defects in mice. In humans, exposure to chloromethane during pregnancy may cause the fetus' lower spinal column, pelvis, and legs to form incorrectly, but this has not been conclusively demonstrated.