Participation of the<i>cis</i>‐12 ethylenic bond to<i>cis‐trans</i> isomerization of the<i>cis</i>‐9 and<i>cis</i>‐15 ethylenic bonds in heated α‐linolenic acid

Abstract

Abstract To understand the cis‐trans isomerization reaction of ethylenic bonds in heated octadecatrienoic acids (occurring during industrial deodorization of oils), we have prepared a mixture of cis ‐9, cis ‐12, cis ‐15, and cis ‐9, cis ‐15 18:2 acids by partial hydrazine reduction of cis ‐9, cis ‐12, cis ‐15 18:3 acid present in linseed oil. This mixture (as fatty acid methyl esters) was heated under vacuum at 270°C for 2.25 h. The two methylene‐interrupted acids isomerize at a similar rate under such conditions, but the nonmethylene‐interrupted cis ‐9, cis ‐15 18:2 acid remains unchanged. This means that the mechanism of isomerization does not involve a direct interaction between the two external ethylenic bonds as previously hypothesized. The central cis ‐12 ethylenic bond is apparently necessary for the isomerization of the two external cis ‐9 and cis ‐15 ethylenic bonds. However, this bond is itself rather protected against isomerization in the original cis ‐9, cis ‐12, cis ‐15 18:3 acid which is mainly isomerized to trans ‐9, cis ‐12, trans ‐15, cis ‐9, cis ‐12, trans ‐15, and trans ‐9, cis ‐12, cis ‐15 18:3 acids. The cis ‐9, trans ‐12, cis ‐15 18:3 isomer is less than 10% of total trans isomers of α‐linolenic acid. As a general rule, only one of the two double bonds in a methylene‐interrupted diethylenic system can undergo cis‐trans isomerization when submitted to heat treatment, at least for temperatures equal to or less than 270°C.