Carbocation rearrangement, methyl migration, and long-range hydrogen transfer in phenyl fatty acid esters

Various isomers can be observed in the Friedel–Crafts alkylation of benzene. However, the mechanism and influence of these isomerization reactions were still unclear. In this work, several methyl phenylundecanoate were analyzed by gas chromatography–mass spectrometry to explore the isomerization of the Friedel–Crafts alkylation. The results showed that there were three isomerization reactions in the Friedel–Crafts alkylation reaction: five isomers of methyl 6–10-phenylundecanoate due to carbocation rearrangement, five isomers of methyl 6–9-methyldecanoate and methyl undecanoate due to methyl migration, and the loss of neutral alcohol molecular fragments due to hydrogen transfer. At higher reaction temperature, carbocation migration was more active and the formation of methyl undecanoate isomers was observed. There was a competitive relationship between the production of methyl undecanoate isomers and methyl phenylundecanoate isomers. In addition, the hydrogen transfer could be initiated in the presence of unsaturated bonds and alcohol molecules lost as a fragment. The hydrogen from the phenyl groups, which were at a different position on the carbon chain of fatty acid esters, could be transferred to the ester group. It was found that hydrogen could be transferred intramolecularly over a long range. This work revealed the isomerization process in the Friedel–Crafts alkylation and further deepened the understanding of alkylation.