Carbonate ester facts for kids

Chemical structure of the carbonate ester group

In organic chemistry, 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 R–O–C(=O)–O–R' and they are related to esters (R–O–C(=O)–R'), ethers (R–O–R') and also to the inorganic carbonates.

Monomers of polycarbonate (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 like dimethyl carbonate, ethylene carbonate, propylene carbonate are used as solvents, dimethyl carbonate is also a mild methylating agent.

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.

In principle carbonate esters can be prepared by direct condensation of methanol and carbon dioxide. The reaction is however thermodynamically unfavorable. A selective membrane can be used to separate the water from the reaction mixture and increase the yield.

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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)

Uses

Organic carbonates are used as solvents in lithium batteries. Due to their high polarity, they dissolve lithium salts. The problem of high viscosity is circumvented by using mixtures for example of dimethyl carbonate, diethyl carbonate, and dimethoxyethane.

They are also used as solvents in organic synthesis. Classified as polar solvents, they 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. Many industrial production pathways for carbonates are not green because they rely on phosgene or propylene oxide.

Dimethyl dicarbonate is commonly used as a beverage preservative, processing aid, or sterilant.