Abstract
AbstractTo understand thecis‐trans isomerization reaction of ethylenic bonds in heated octadecatrienoic acids (occurring during industrial deodorization of oils), we have prepared a mixture ofcis‐9,cis‐12,cis‐15, andcis‐9,cis‐15 18:2 acids by partial hydrazine reduction ofcis‐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‐interruptedcis‐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 centralcis‐12 ethylenic bond is apparently necessary for the isomerization of the two externalcis‐9 andcis‐15 ethylenic bonds. However, this bond is itself rather protected against isomerization in the originalcis‐9,cis‐12,cis‐15 18:3 acid which is mainly isomerized totrans‐9,cis‐12,trans‐15,cis‐9,cis‐12,trans‐15, andtrans‐9,cis‐12,cis‐15 18:3 acids. Thecis‐9,trans‐12,cis‐15 18:3 isomer is less than 10% of totaltrans isomers of α‐linolenic acid. As a general rule, only one of the two double bonds in a methylene‐interrupted diethylenic system can undergocis‐trans isomerization when submitted to heat treatment, at least for temperatures equal to or less than 270°C.
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