Propylene glycol

Propylene glycol, also called propane-1,2-diol, is an organic compound with the chemical formula C₃H₈O₂. It is a viscous colourless liquid which is nearly odourless but possesses a faintly sweet taste. Chemically it is classed as a diol and is miscible with a broad range of solvents, including water, acetone, and chloroform.

It is produced on a large scale and is primarily used in the production of polymers but also sees use in food processing as the E-number E1520.

The compound is sometimes called α-propylene glycol to distinguish it from the isomer propane-1,3-diol (β-propylene glycol).

Structure and properties

Propylene glycol is a clear, colorless and hygroscopic liquid. Propylene glycol contains an asymmetrical carbon atom, so it exists in two enantiomers. The commercial product is a racemic mixture. Pure optical isomers can be obtained by hydration of optically pure propylene oxide.

The freezing point of water is depressed when mixed with propylene glycol owing to the effects of dissolution of a solute in a solvent (freezing-point depression); in general, glycols are non-corrosive, have very low volatility and very low toxicity, however, the closely related ethylene glycol (a key ingredient in antifreeze) is moderately toxic to humans and fatally toxic to many animals.