A carbonate ester (organic carbonate or organocarbonate) is an ester of carbonic acid. This functional group consists of a carbonyl group flanked by two alkoxy groups. The general structure of these carbonates is R1O(C=O)OR2 and they are related to esters R1O(C=O)R and ethers R1OR2 and also to the inorganic carbonates.
Monomers of polycarbonate (e.g. Lexan) are linked by carbonate groups. These polycarbonates are used in eyeglass lenses, compact discs, and bulletproof glass. Small carbonate esters like dimethyl carbonate, and ethylene and propylene carbonate are used as solvents. Dimethyl carbonate is a mild methylating agent as well.
The chemistry of carbonate esters has been reviewed.
Carbonate esters can be divided into three categories by their structures. The first and general case is the dialkyl or diaryl carbonate that comprises a carbonate group with two R substituents. The simplest members of this class include dimethyl carbonate and diphenyl carbonate:
Instead of terminal alkyl or aryl R-groups, two carbonate groups can be linked by an aliphatic or aromatic bifunctional group. For example, poly(propylene carbonate) and poly(bisphenol A carbonate) (Lexan):
Poly(bisphenol A carbonate) (Lexan)
There are two main ways of preparing carbonate esters: the reaction of an alcohol (or phenol) with phosgene (phosgenation), and the reaction of an alcohol with carbon monoxide and an oxidizer (oxidative carbonylation). Other carbonate esters may subsequently be prepared by transesterification.
Alcohols react with phosgene to yield carbonate esters according to the following reaction:
- 2 ROH + COCl2 → ROCO2R + 2 HCl
Phenols react similarly, and bisphenol A polycarbonates are produced in this manner. This process is high yielding. However, toxic phosgene is used, and stoichiometric quantities of base (e.g. pyridine) are to neutralize the hydrochloric acid formed. This process is undesirable as a result.
Chloroformate esters are intermediates in this process. Rather than reacting with additional alcohol, they may disproportionate to give the desired carbonate diesters and one equivalent of phosgene. Half the amount of phosgene and base are needed, and half the salt waste is generated:
- PhOH + COCl2 → PhOCOCl + HCl
- 2 PhOCOCl → PhOCO2Ph + COCl2
Oxidative carbonylation 
- 2 CH3OH + CO + [O] → CH3OCO2CH3 + H2O
Reaction of carbon dioxide with epoxides 
The reaction of carbon dioxide with epoxides is a general route to the preparation of cyclic 5-membered carbonates. Annual production of cyclic carbonates was estimated at 100,000 tonnes per year in 2010. Industrially, ethylene and propylene oxides readily react with carbon dioxide to give ethylene and propylene carbonates (with an appropriate catalyst). For example:
- C2H4O + CO2 → C2H4O2CO
Catalysts for this reaction have been reviewed, as have non-epoxide routes to these cyclic carbonates.
Carbonate transesterification 
Once the initial carbonate has been produced, it may be converted to other carbonates by transesterification. A more nucleophilic alcohol will displace a less nucleophilic alcohol. In other words, aliphatic alcohols will displace phenols from aryl carbonates. If the departing alcohol is more volatile, the equilibrium may be driven by distilling that off.
Organic synthesis 
Laboratory methods for the synthesis of carbonate ester in the laboratory are:
- from the corresponding diols.
- by double oxidation of ketones in a Baeyer-Villiger rearrangement,although this method is plausible,there is no evidence that it is effective.
- by reaction of an epoxide with carbon dioxide catalysed by a zinc halide 
Use as solvents 
A large number of organic carbonates are used as solvents. They are classified as polar solvents and have a wide liquid temperature range. One example is propylene carbonate with melting point −55 °C and boiling point 240 °C. Other advantages are low ecotoxicity and good biodegradability. The industrial production of carbonates is not green because methods for production of linear carbonates rely on phosgene and those of cyclic carbonates rely on propylene oxide.
Use in batteries 
Organic carbonates are used as a solvent in lithium batteries; due to their high polarity they can dissolve lithium salts. The problem of high viscosity is circumvented by using carbonate mixtures for example mixtures of dimethyl carbonate, diethyl carbonate and dimethoxy ethane.
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