Sulfonation Mechanism of Fatty Acid Methyl Ester with Sulfur Trioxide

Abstract

Methyl laurate used in this study as an original fatty ester) was easily converted to a 1 molar SO₃ adduct (first intermediate) by adding SO₃ at-20°C. This intermediate was found to be in equilibrium with the original ester. The energy of addition of SO₃ to form the intermediate was about 14 kcal/mol based on exchange rates of the equilibrium.
Sulfonation of the α-position of an ester occurred not on the original ester but on the first intermediate to give a 2-molar SO₃ adduct (second intermediate). The α-sulfonation followed kinetically the second order for the first intermediate and had an activation energy of about 25 kcal/mol.
The rate of the final reaction to the α-sulfofatty acid methyl ester (α SFM) from the second intermediate depended on the amount of co-existing ester.
According to the experimental results, the sulfonation mechanism for α SFM is proposed.