toxic phosgene is used, and stoichiometric quantities of base (e.g. pyridine) are required to neutralize the hydrogen chloride that is cogenerated. 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: 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: 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. 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 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. 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). This process is not an economical. Reactions Carbonate esters undergo many of the reactions of conventional carboxylic acid esters. With Grignard reagents carbonate esters react to give tertiary alcohols. Some cyclic carbonates are susceptible to polymerization. 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