Insight into the shift and rearrangement of carbocation in Friedel-Crafts alkylation of unsaturated fatty acids revealed by GC–MS

• The shift of carbocation was found from Friedel-Crafts alkylation of oleic acid by GC–MS.
• The product of Friedel-Crafts alkylation of oleic acid was 11 iosmers.
• Intramolecular quenching could stop the carbocation shifting.

We observed that Friedel-Crafts alkylation of unsaturated fatty acids yielded diverse products because of the shift and rearrangement of carbocation. However, these shifts and rearrangement rules of carbocation in Friedel-Crafts alkylation were unclear. For a better understanding of these rules, the product diversity of Friedel-Crafts alkylation of oleic acid with phenyl was studied by GC–MS. The product structures were identified by the characteristic m/z obtained in electron impact ionization and the relative contents of diverse isomers were calculated by the normalization method of integration areas in extracted ion and total ion chromatograms. The results suggested there were 11 isomeric compounds, 7–17- phenyl stearic acids with relative contents ranging from 2.9% to 16.4%. The inhibition factor of the shift and rearrangement of carbocation was also being investigated. The carbocation was easily quenched by oxygen-containing compounds such as ester, ether, and alcohol. Intramolecular quenching stopped the carbocation shifting and inhibited 2–6- phenyl stearic acids formation. The study contributed to the understanding of the shift and rearrangement of carbocation, diversity of produced isomers and inhibitors in Friedel-Crafts alkylation of the unsaturated fatty acids, which were helpful in preparing and manufacturing relative unsaturated fatty acids by alkylation.

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