Instability of Propyl Lactate Prepared by Boron Trifluoride-Catalyzed Esterification of Lactic Acid

The official first action gas chromatographic method for the determination of lactic, β-hydroxyhutyric, and succinic acids, 17.034–17.038, requires the formation of their propyl esters with boron trifluoride-1-propanol and extraction of the esters into an organic solvent prior to chromatography. Previously published data showed that the solutions of propyl esters were stable for as long as 4 weeks when they were extracted into chloroform and stored at 40°F. However, when ethyl ether is substituted for chloroform, propyl lactate is no longer stable during storage. The ether apparently extracts some unreacted boron trifluoride which eventually causes the hydrolysis of propyl lactate. The propyl esters of β-hydroxybutyric, levulinic, and succinic acids were stable in ether as well as in chloroform. The method has been revised to include the use of ethyl ether, if required by the gas chromatographic system, and to specify analysis of the ether solutions within 24 hr of esterification.