Dimethyl carbonate

Production

Dimethyl carbonate can be prepared by the reaction of phosgene with methanol via methyl chloroformate:

COCl₂ + CH₃OH → CH₃OCOCl + HClCH₃OCOCl + CH₃OH → CH₃OCO₂CH₃ + HCl

Overall:

COCl₂ + 2 CH₃OH → CH₃OCO₂CH₃ + 2 HCl

This historical synthesis process is undesirable because of phosgene's toxicity and has been largely replaced by other synthesis routes. It is produced industrially by a transesterification of ethylene carbonate or propylene carbonate and methanol, which also affords respectively ethylene glycol or propylene glycol. A second large scale commercial synthesis route uses a process where carbon monoxide, methanol and oxygen are reacted to form dimethyl carbonate. World production in 1997 was estimated at 1000 barrels a day. Production of dimethyl carbonate worldwide is limited to Asia, the middle east and Europe.

Methylating agent

Dimethyl carbonate methylates anilines, phenols and carboxylic acids, but many of these reactions require the use of an autoclave.

Dimethyl carbonate's main benefit over other methylating reagents such as iodomethane and dimethyl sulfate is its much lower toxicity and its biodegradability. Unfortunately, it is a relatively weak methylating agent compared to these traditional reagents.

Solvent

In the US, DMC was exempted under the definition of volatile organic compounds by the U.S. EPA in 2009. Due to its classification as VOC exempt, dimethyl carbonate has grown in popularity and applications as a replacement for methyl ethyl ketone, tert-butyl acetate, and parachlorobenzotrifluoride. DMC has an ester or alcohol like odor, which is more favorable to users than most hydrocarbon solvents it replaces. DMC has an evaporation rate of 3.22 (butyl acetate = 1.0), which slightly slower than MEK (3.8) & Ethyl Acetate (4.1) and faster than toluene (2.0) & isopropanol (1.7). DMC has solubility profile similar to common glycol ethers, meaning DMC can dissolve most common coating resins except perhaps rubber based resins. Hildebrand solubility parameter is 20.3 Mpa and Hansen solubility parameters are: dispersion = 15.5, polar = 3.9, H bonding = 9.7. DMC is partially soluble in water up to 13%, however DMC is hydrolyzed in water based systems over time to methanol and CO₂ unless properly buffered. DMC can freeze at same temperatures as water, it can be thawed out with no loss of properties to itself or coatings based on DMC.

Intermediate in polycarbonate synthesis

A very large captive use of dimethylcarbonate is for the production of diphenylcarbonate through transesterification with phenol. Diphenylcarbonate is a widely used raw material for the synthesis of Bisphenol-A-polycarbonate in a melt polycondensation process. the most common route is described by Fukuoka, of the Japanese "Asahi Kasei" chemical corporation.

Alternative fuel additive

There is also interest in using this compound as a fuel oxygenate additive.

Safety

DMC is a flammable liquid that has a flash point of 17 °C (63 °F), which would limit its use in consumer and indoor applications. DMC is still safer than acetone, methyl acetate and methyl ethyl ketone from a flammability point of view. DMC has a recommended industrial exposure (REL) limit of 100 ppm by inhalation over an 8 hour work day, which is similar to a number of common industrial solvents (toluene, methyl ethyl ketone). Workers should wear protective organic vapor respirators when using DMC indoors or in other conditions where concentrations exceed the REL. DMC is metabolized by the body to methanol and carbon dioxide, so accidental ingestion should be treated in the same manner as methanol poisoning.