Carbonate ester
In
Structures
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 and diphenyl carbonate.
Alternatively, the carbonate groups can be linked by a 2- or 3-carbon bridge, forming cyclic compounds such as ethylene carbonate and trimethylene carbonate. The bridging compound can also have substituents, e.g. CH3 for propylene 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 as
Preparation
Organic carbonates are not prepared from inorganic carbonate salts. Two main routes to carbonate esters are practiced: 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.[2][3]
In principle carbonate esters can be prepared by direct condensation of methanol and carbon 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]
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Diphenyl carbonate, a representative acyclic carbonate ester
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Dimethyl dicarbonate, a preservative
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Ethylene carbonate, a cyclic carbonate ester
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Trimethylene carbonate, another cyclic carbonate ester
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Poly(propylene carbonate)
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Poly(bisphenol A carbonate), a commercially important plastic (Lexan)
Phosgenation
Alcohols react with phosgene to yield carbonate esters according to the following reaction:
- 2 ROH + COCl2 → ROC(O)OR + 2 HCl
Phenols react similarly. Polycarbonate derived from bisphenol 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]
- PhOH + COCl2 → PhOC(O)Cl + HCl
- 2 PhOC(O)Cl → PhOC(O)OPh + COCl2
Overall reaction is:
- 2 PhOH + COCl2 → PhOC(O)OPh + 2 HCl
Oxidative carbonylation
Oxidative carbonylation is an alternative to phosgenation. The advantage is the avoidance of phosgene. Using copper catalysts, dimethylcarbonate is prepared in this way:[3][9]
- 2 MeOH + CO + 1/2 O2 → MeOC(O)OMe + H2O
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]
Reaction of carbon dioxide with epoxides
The reaction of carbon dioxide with
- C2H4O + CO2 → C2H4O2CO
Carbonate transesterification
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]
Reactions
Carbonate esters undergo many of the reactions of conventional carboxylic acid esters. With
Uses
Organic carbonates are used as
They are also used as solvents in organic synthesis.
Dimethyl dicarbonate is commonly used as a beverage preservative, processing aid, or sterilant.[14]
References
- ^ .
- ^ PMID 11848777.
- ^ ISBN 978-3527306732.
- .
- .
- ^ Vermerris, René. Membrane enhanced conversion of methanol and carbon dioxide into dimethyl carbonate (PDF) (Report). Archived (PDF) from the original on 2013-10-05.
- PMID 20480023.
- .
- PMID 11848777.
- ISBN 9783527337811.)
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: CS1 maint: numeric names: authors list (link - .
- PMID 20345182.
- ^ Sibiya, Mike Sbonelo (19 May 2008). Catalytic transformation of propylene carbonate into dimethyl carbonate and propylene glycol (Master of Science in Chemistry thesis). University of Johannesburg. Archived from the original on 2020-08-18. Retrieved 2022-04-12.
- ISBN 978-3527306732.