Conversion of conjugatedcis‐trans octadecadienoates to thetrans‐trans isomers1,2,3

Summary The conversion of methyl conjugatedcis‐trans octadecadienoate to thetrans‐trans isomer takes place easily in the presence of iodine and light. The reaction is more rapid in carbon disulfide than in Skellysolve C, and it is quite slow and accompanied by loss of conjugation in methanol. The rate of conversion increases with iodine concentration and light intensity and decreases with increasing fat concentration. It is affected by temperature also, but to a lesser degree. the influence of light may result from its catalytic effect on the decomposition of a reversible fat‐iodine complex. The reaction appears to be pseudo‐first order with respect to the substrate but is complicated by side reactions which inactivate the catalyst and destroy conjugation. The reaction occurs, at a measurable rate, with iodine in the dark, or in light without iodine, but no conversion takes place without iodine in darkness. The isomerization reaches an equilibrium when approximately 71% of the conjugation present is in thetrans‐trans form. This equilibrium can be attained by starting with eithercis‐trans ortrans‐trans material. Very little, if any,cis‐cis isomer is present in the equilibrium mixture.