Carbonate ester

Chemical structure of the carbonate ester group.

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.^[1]

Types

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:

• 
  Dimethyl carbonate
• 
  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(propylene carbonate)
• 
  Poly(bisphenol A carbonate) (Lexan)

Alternatively, the carbonate groups can be linked by a 2- or 3-carbon bridge, such as ethylene carbonate and trimethylene carbonate; substituents, e.g. CH₃ for propylene carbonate are possible too:

• 
  Ethylene carbonate
• 
  Trimethylene carbonate

Oleochemical carbonates

Carbonates (esters of carbonic acid, H₂CO₃) are well known to chemists as they represent an important class of organic compounds and among them oleochemical carbonates have interesting characteristics which make them candidates for many industrial applications.

The most common carbonates have the following structure: RO—CO—OR. R is a linear chain with 8 to 18 carbon atoms, saturated or with one double bond (dioleyl carbonate), or a branched chain (ethylhexyl, butyloctyl, or hexyldecyl).

They are miscible in organic solvents but insoluble in water. Unsaturation or branching on the alkyl chain lowers their melting point.^[2] The condensation of phosgene (ClCOCl) with an alcohol appears the most commonly used procedure to synthesize oleochemical carbonates.

The polar nature of the carbonate moiety enables it to adhere strongly to metal surfaces. Thus, they are used as lubricant components which have a protective property for metal corrosion. Some C8 to C18 carbonates have been exploited in personal-care products (sunscreen, cosmetics), dioctyl carbonate being also used as emollient or solvent in UV-filter solutions. Extraction of metal ions (gold, silver, platinum) is improved by the use of the chelating properties of oleochemical carbonates when mixed with the metal-containing aqueous phase. Future developments will ensure a growing interest in these molecules.

Preparation

There are two main industrial 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.^[1][3]

Phosgenation

Alcohols react with phosgene to yield carbonate esters according to the following reaction:

2 ROH + COCl₂ → ROCO₂R + 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.^[1][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 → PhOCOCl + HCl

2 PhOCOCl → PhOCO₂Ph + COCl₂

Oxidative carbonylation

With an appropriate catalyst, alcohols can react with carbon monoxide and an oxidant (e.g. oxygen gas):^[1][3]

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.^[4] Industrially, ethylene and propylene oxides readily react with carbon dioxide to give ethylene and propylene carbonates (with an appropriate catalyst).^[1][3] For example:

C₂H₄O + CO₂ → C₂H₄O₂CO

Catalysts for this reaction have been reviewed, as have non-epoxide routes to these cyclic carbonates.^[4]

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.^[1][3]

Experimental routes

From carbon dioxide and alcohols

In principle carbonate esters can be prepared the direct condensation of methanol and carbon dioxide.^[5][6] The reaction is thermodynamically unfavorable, due to the buildup of water byproduct.^[7] A selective membrane can be used to separate the water from the reaction mixture and increase the yield.^[8][9]

From urea with alcohols

Dimethyl carbonate can be made from the reaction of methanol with urea. Ammonia that is produced can be recycled. Effectively ammonia serves as a catalyst for the synthesis of dimethyl carbonate. The byproducts are methyl- and N-methylcarbamate (the latter from the reaction between dimethyl carbonate and methyl carbamate). The yield of dimethyl carbonate is only 30%, so unless the byproducts are recycled or commercially realized this is not an economic method.^[10]

Organic synthesis

Laboratory methods for the synthesis of carbonate ester in the laboratory are from the corresponding diols, or by reaction of an epoxide with carbon dioxide catalysed by a zinc halide.^[11]

Use as solvents

A large number of organic carbonates are used as solvents.^[12] 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. Many industrial production pathways for carbonates are not green because they rely on phosgene or propylene oxide.^[13]

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.

References