Inorganic chemistry, acarbonate ester (organic carbonate ororganocarbonate) is anester ofcarbonic acid. Thisfunctional group consists of acarbonyl group flanked by twoalkoxy groups. The general structure of these carbonates isR−O−C(=O)−O−R′ and they are related toesters (R−O−C(=O)−R′),ethers (R−O−R′) and also to the inorganiccarbonates.
Monomers ofpolycarbonate (e.g. Makrolon or Lexan) are linked by carbonate groups. These polycarbonates are used in eyeglass lenses, compact discs, and bulletproof glass. Small carbonate esters likedimethyl carbonate,ethylene carbonate,propylene carbonate are used as solvents, dimethyl carbonate is also a mildmethylating agent.
Carbonate esters have planar OC(OC)2 cores, which confers rigidity. The unique O=C bond is short (1.173 Å in the depicted example), while the C–O bonds are more ether-like (the bond distances of 1.326 Å for the example depicted).[1]
Carbonate esters can be divided into three structural classes: acyclic, cyclic, and polymeric. The first and general case is the acyclic carbonate group. Organic substituents can be identical or not. Both aliphatic or aromatic substituents are known, they are called dialkyl or diaryl carbonates, respectively. The simplest members of these classes are dimethyl carbonate anddiphenyl carbonate.
Alternatively, the carbonate groups can be linked by a 2- or 3-carbon bridge, forming cyclic compounds such as ethylene carbonate andtrimethylene carbonate. The bridging compound can also have substituents, e.g. CH3 forpropylene carbonate. Instead of terminal alkyl or aryl groups, two carbonate groups can be linked by an aliphatic or aromatic bifunctional group.
A third family of carbonates are the polymers, such aspoly(propylene carbonate) andpoly(bisphenol A carbonate) (e.g. Makrolon or Lexan).
Two main routes to carbonate esters are practiced: the reaction of an alcohol (or phenol) withphosgene (phosgenation), and the reaction of an alcohol withcarbon monoxide and an oxidizer (oxidative carbonylation). Other carbonate esters may subsequently be prepared bytransesterification.[2][3]
In principle dimethylcarbonate can be prepared by directcondensation ofmethanol andcarbon dioxide. The reaction is however thermodynamically unfavorable.[4] A selective membrane can be used to separate the water from the reaction mixture and increase the yield.[5][6][7][8]
Alcohols react with phosgene to yield carbonate esters according to the following reaction:
Phenols react similarly.Polycarbonate derived frombisphenol A is produced in this manner. This process is high yielding. However, toxic phosgene is used, and stoichiometric quantities of base (e.g. pyridine) are required to neutralize the hydrogen chloride that is cogenerated.[2][3] 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:[3]
Overall reaction is:
Oxidative carbonylation is an alternative to phosgenation. The advantage is the avoidance ofphosgene. Using copper catalysts, dimethylcarbonate is prepared in this way:[3][9]
Diphenyl carbonate is also prepared similarly, but using palladium catalysts. The Pd-catalyzed process requires a cocatalyst to reconvert the Pd(0) to Pd(II). Manganese(III) acetylacetonate has been used commercially.[10]
The reaction of carbon dioxide withepoxides 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.[11] Industrially, ethylene and propylene oxides readily react with carbon dioxide to give ethylene and propylene carbonates (with an appropriate catalyst).[2][3] For example:
Carbonate esters can 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.[2][3]
Carbonate esters undergo many of the reactions of conventional carboxylic acid esters. WithGrignard reagents carbonate esters react to givetertiary alcohols. Some cyclic carbonates are susceptible to polymerization.
Pyrolysis of poly(1,6-hexylenecarbonate) gives 5-hexenal as described by the following idealized equation:[3]
The corresponding decenol is produced from the polycarbonate derived from 1,10-decanediol.
Organic carbonates are used assolvents inlithium batteries. Due to their high polarity, they dissolve lithium salts. The problem of high viscosity is circumvented by using mixtures for example ofdimethyl carbonate,diethyl carbonate, anddimethoxyethane.
They are also used as solvents in organic synthesis.[12] Classified aspolar solvents, they have a wide liquid temperature range. One example is propylene carbonate withmelting point −55 °C and boiling point 240 °C. Other advantages are lowecotoxicity and goodbiodegradability. Many industrial production pathways for carbonates are not green because they rely onphosgene orpropylene oxide.[13]
Dimethyl dicarbonate is commonly used as abeveragepreservative, processing aid, orsterilant.[14]
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