An improved procedure is described for the preparation of pure linolenic acid from linseed oil. Crystallization of urea inclusion compounds is used to produce a concentrate of methyl linolenate. Adsorption chromatography then is employed to provide fractions with a purity of approximately 99 ~. After fractional distillation, concentrates of linolenic acid are purified further by repeated crystallizations at low temperature. Linolenic acid and its methyl ester have been prepared generally via their hexabromidesl,Z). However, the bromination-debromination technique leads to considerable inversion of double bonds4-6). Crystallization of inclusion compounds a) and low temperature crystallization 7) also have been employed for the isolation of linolenic acid, but these techniques have not yielded preparations purer than 90%. Matthews et al. 4) prepared pure linolenic acid by means of low temperature crystallization of the mixture of cis-trans isomers obtained via the bromination-debromination method. Pure methyl linolenate has also been prepared by countercurrent distribution 8, 9) and by liquid-liquid partition chromatography 10).
The latter two methods yield linolenic acid of high purity, but they are tedious and are not applicable to the isolation of large quantities. This communication describes a more efficient procedure for the preparation of pure linolenic acid. The method involves a combination of crystallization of urea inclusion compounds, adsorption chromatography, fractional distillation, and crystallization at low temperatures.
Experimental
Materials
Crude linseed oil was purchased from Archer Daniels Midland Company,