Isomerization of linoleic, linolenic, and other polyunsaturated acids with potassium tertiary butoxide and its application in the spectrophotometric estimation of these acids

Abstract

SummaryIt has been shown that potassium tertiary butoxide isomerizes such unsaturated fatty acids as linoleic, linolenic, arachidonic, and pentaenoic at a temperature of 90°. With 5% potassium‐t‐butanol reagent conjugation of linoleic acid attains a maximum at the end of 2 hrs. and remains steady thereafter even up to 10 hrs. while with linolenic, arachidonic, and pentaenoic acids it is still occurring at the end of such long times of reaction as 48 and 10 hrs., respectively. This continuance of reaction at the end of such long periods with these higher unsaturated acids demonstrates that the isomerization with the tertiary butoxide reagent at 90° is not complicated by side reactions, such as polymerization and deisomerization. All the methods studied give high absorption coefficients in the lower regions of maxima—233 mµ with linolenic, 233 mµ and 268 mµ with arachidonic acids—showing thereby that isomerizations of these acids occur stepwise. Based on this study, a method using 5% potassium‐t‐butanol as reagent, at 90° with a time of reaction of 4 hrs., has been proposed for the estimation of linoleic and linolenic acids. The k233 value for linoleic acid (94.0) obtained by this method compares very well with those by the 6.6% KOH‐glycol and 21% KOH‐glycol methods while with linolenic acid the k233 value (63.2) is higher and k268 value (74.2) lies intermediate between those by the other methods. The proposed method is shown to estimate the linoleic and linolenic acid contents of several typical oils with about the same degree of accuracy as either the 6.6% KOH‐glycol or 21% KOH‐glycol methods.Appendix. When this paper was ready to be submitted, a note (27) describing the use of tertiary butoxide reagent on the kinetic study of isomerization of linoleic and linolenic acids came to our attention. Extinction coefficients at 235 mµ and 268 mµ are reported for linolenic acid and the values (61.4, 74.2) obtained by isomerization with a molar solution of the reagent at 99.5° and 180 minutes’ time of reaction are about the same order as those reported in this paper. It is of interest to note that these authors have also noted that no destruction of the product occurs at this temperature and time of heating.