<i>Cis‐trans</i> isomerization of oleic, linoleic and linolenic acids

Abstract

Abstract The equilibrium composition of cis and trans isomers obtained by isomerizing oleic, linoleic, and linolenic acids with selenium or nitrous acid has been studied using gas chromatography and infrared spectroscopy. The oleic/elaidic equilibrium mixture was found to contain 75舑80% elaidic acid instead of the generally accepted 66% value. It is felt that the greater accuracy of gas chromatography and infrared analyses over older methods allows this equilibrium to be defined with greater precision. Similar studies on the cis‐trans isomerization of linoleic and linolenic acids indicated that their equilibrium mixtures also contained 75舑80% trans double bonds. With linoleic acid, these trans bonds were shown to be randomly distributed among the double bonds present. Cis‐trans isomerization of linoleic or linolenic acids with selenium produced by‐products having elution times equivalent to 18蝘2, 18蝘1, and 18蝘0 on a gas chromatograph. No such by‐products were observed when oleic acid was isomerized. Apparently some type of hydrogen‐transfer reaction accompanies the cis‐trans isomerization of polyunsaturated acids with selenium.