Propylene Carbonate as a Useful Solvent for Organic Synthesis Processes

The density of propylene carbonate is 1.189 g/cc and the dielectric constant and dipole moment respectively are 64.94 and 16.5.  It is a solvent that can be expected to provide good solubility for a wide range of organic reaction substrates.

Propylene carbonate may be the only solvent that is (a) usefully immiscible with water, (b) does not contain a halogen in its formula, yet (c) has a density greater than water. At 25°C the solubility of propylene carbonate in water is 8.3% and the solubility of water in propylene carbonate is 17.5%. Excess water forms a second phase on top of the water-saturated propylene carbonate. When the mixture of liquids is cooled to near 0° C the separation of phases is even greater. Thus, propylene carbonate can provide two phases that can be used for liquid-liquid extractions. Moreover, the more predominantly organic layer is the lower phase and in a reactor can be simply cut through the bottom valve. Thus, when the product can be taken into the aqueous phase by acid or base, the organic phase can be removed leaving the product in the reactor. This can save a vessel in a chemical process work-up.

 In fact, propylene carbonate is thermotropic with appropriate water mixtures. As the temperature is varied between 0 and 61°C, the two phases that derive from a particular weight fraction of propylene carbonate and water, change compositions. It would therefore be expected that the partitioning of a mixture of substrates, such as might be the products from a reaction step,  could be optimized between the two phases both by varying the propylene carbonate/water weight fraction and by changing the temperature of the two-phase mixture. The UCST for propylene carbonate and water is about 72 C. At this temperature, only a single distinct clear phase remains. 

Propylene carbonate can be hydrolyzed by both aqueous acid and aqueous base. There is both a good and a bad aspect to this. The bad is that the stability of the solvent in contact with water is 

somewhat limited. This, however, is also true of ethyl acetate where it is not regarded as a severe limitation. The good part is that small amounts of the solvent mixed with a hydrolytically stable cosolvent can be removed by hydrolysis since the products, carbon dioxide and propylene glycol are both water-soluble.