Dimer acid structures. The dehydro‐dimer from methyl oleate and Di‐t‐butyl peroxide

Abstract

Abstract The dehydro‐dimer of methyl oleate was prepared and its structure determined as a model of a non‐ring dimer for reference in studying the structure of other fatty dimer acids. The dehydro‐dimer of methyl oleate is formed by the action of di‐t‐butyl peroxide on methyl oleate. The reaction is stoichiometric; one mole of DTBP producing one mole of dehydrodimer and two moles of t‐BuOH, when excess methyl oleate is used. The dimer was shown to contain two double bonds, and to be formed by carbon‐to‐carbon linkages predominantly and equally at the 8, 9, 10 and 11 carbons of the oleate monomer segments. Unsaturation was determined by quantitative hydrogenation and far UV absorption. The points of linkage were established by diagnosis of the positions of the involved tertiary carbons of the hydrogenated dimer 1) by chemical oxidation, and 2) by mass spectrometry. Positions of the double bonds were determined by quantitative ozonization, reductive cleavage followed by gas chromatography of the aldehydes and aldehyde esters. Precise molecular weight of the hydrogenated dimer was determined from the parant mass peak at the expected m/e of 594, confirming the non‐ring structure. The unhydrogenated dimer showed a parent m/e peak at the expected value of 590. The bridging at the 8 and 10 positions is explained as being due to coupling of radicals with limiting resonance structures resulting from loss of a hydrogen atom from the methylene at position 8. The bridging at the 9 and 11 positions is explained as due to coupling of limiting resonance structures resulting from loss of a hydrogen atom from the methylene at position 11. Mass spectrometric data indicate that the dimerization is a coupling of the expected free radical forms, rather than attack by an oleate free radical on the double bond of an intact oleate molecule, with subsequent loss of hydrogen to form the second double bond in the dimer. Coupling at the 2‐position (ॅ to COOCH 3 ) occurs in not more than 5舑10% of the molecules. A small amount of cyclic dimer may be present.