Synthesis of Mono-, Di- and Triisopropyl Citrates and Analysis of Chemical Structures of Their Isomers by ¹H- and ¹³C-NMR Spectra

Abstract

In order to obtain mono-, di- and triisopropyl citrate as standard of food additives, their syntheses were studied. On refluxing anhydrous citric acid (I) and isopropanol with a small amount of sulfuric acid, five kinds of esters, monoesters (IIa and IIb), diesters (IIIa and IIIb) and triester (IV) were obtained, and IIb, IIIb and IV were isolated. On the other hand, alkaline hydrolysis of IV gave I, IIa and IIIa, and IIa and IIIa were isolated. These compounds II, III and IV were distinguished by gas chromatography and mass spectrometry, but chemical structures of isomers of monoesters (IIa and IIb) and diesters (IIIa and IIIb) could not be revealed. The study on ¹H- and ¹³C-NMR spectra made it clear that IIa, IIb, IIIa and IIIb were 2-monoisopropyl citrate, 1-monoisopropyl citrate, 1, 2-diisopropyl citrate and 1, 3-diisopropyl citrate, respectively. Recovery of 100mg/l of our authentic each isopropyl citrate (IIa, IIb, IIIa, IIIb and IV) from edible oil by our established method was more than 96%.