Chloromethane

Production

Chloromethane (originally called "chlorohydrate of methylene") was among the earliest organochlorine compounds to be discovered when it was synthesized by French chemists Jean-Baptiste Dumas and Eugène-Melchior Péligot in 1835 by boiling a mixture of methanol, sulfuric acid, and sodium chloride. This method is the forerunner for that used today, which uses hydrogen chloride instead of sulfuric acid and sodium chloride.

Chloromethane is produced commercially by treating methanol with hydrochloric acid or hydrogen chloride, according to the chemical equation:

CH₃OH + HCl → CH₃Cl + H₂O

A smaller amount of chloromethane is produced by treating a mixture of methane with chlorine at elevated temperatures. This method, however, also produces more highly chlorinated compounds such as dichloromethane, chloroform, and carbon tetrachloride. For this reason, methane chlorination is usually only practiced when these other products are also desired. This chlorination method also cogenerates hydrogen chloride, which poses a disposal problem.

CH₄ + Cl₂ → CH₃Cl + HCl

CH₃Cl + Cl₂ → CH₂Cl₂ + HCl

CH₂Cl₂ + Cl₂ → CHCl₃ + HCl

CHCl₃ + Cl₂ → CCl₄ + HCl

Dispersion in the environment

Most of the methyl chloride present in the environment ends up being released to the atmosphere. After being released into the air, the atmospheric lifetime of this substance is about 10 months with multiple natural sinks, such as ocean, transport to the stratosphere, soil, etc.

On the other hand, when the methyl chloride emitted is released to water, it will be rapidly lost by volatilization. The half-life of this substance in terms of volatilization in the river, lagoon and lake is 2.1 h, 25 h and 18 days, respectively.

The amount of methyl chloride in the stratosphere is estimated to be 2×10⁶ tonnes per year, representing 20–25% of the total amount of chlorine that is emitted to the stratosphere annually.

Uses

Large scale use of chloromethane is for the production of dimethyldichlorosilane and related organosilicon compounds. These compounds arise via the direct process. The relevant reactions are (Me = CH₃):

x MeCl + Si → Me₃SiCl, Me₂SiCl₂, MeSiCl₃, Me₄Si₂Cl₂, ...

Dimethyldichlorosilane (Me₂SiCl₂) is of particular value as a precursor to silicones, but trimethylsilyl chloride (Me₃SiCl) and methyltrichlorosilane (MeSiCl₃) are also valuable. 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, e.g. the production of methylcellulose. 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.

Obsolete applications

Chloromethane was widely used as a refrigerant during the 1920s and 1930s, before being replaced by safer alternatives such as chlorofluorocarbons and hydrofluorocarbons. In the late 1920s, some manufacturers promoted chloromethane as a safer and less odorous option compared to sulfur dioxide and ammonia. However, a series of fatal leaks in 1928 and 1929 raised serious concerns related to its toxicity and flammability. Although chloromethane has a faint sweet odor, its subtle scent made leaks difficult to detect. To address this issue, acrolein was later added as a nasal-irritating tracer, enhancing leak detection and serving as a warning mechanism.

Chloromethane was also once used for producing the lead-based gasoline additives tetramethyllead and tetraethyllead.

During the late 1880s, chloromethane began to be employed in a limited extent for its anesthetic properties, serving as both a general and local agent. Its use extended into the late 1920s as a component of the inhalation anesthetic Somnoform.

Toxicity

Following inhalation, chloromethane is rapidly absorbed into the bloodstream and distributed throughout the body's tissues. It is primarily metabolized through conjugation with glutathione by the GST enzyme. This process depletes cellular glutathione levels and transforms the chloromethane into toxic intermediates, including formaldehyde and formate. These products are eventually integrated into the "one-carbon pool" for biosynthesis or exhaled as carbon dioxide. A secondary, less dominant metabolic pathway involves oxidation by cytochrome P450.

Methyl chloride is considered to act as a significant central nervous system (CNS) depressant. Acute inhalation produces effects similar to alcohol intoxication, including respiratory depression, dizziness, blurred vision, lack of coordination, and slurred speech; severe cases can progress to convulsions, coma, and death. Occupational safety standards have established a threshold limit value (TLV) and a maximum allowable concentration (MAC) of 50 ppm as a time-weighted average to mitigate these risks. Prolonged or repeated exposure has been linked to potential mutagenic effects and chronic organ toxicity in animal models. However, the EPA classifies its carcinogenic potential in humans as "cannot be determined" due to species-specific metabolic differences (specifically the activity of the CYP2E1 enzyme in mouse kidneys but not in humans).