The mechanism of the formation of sodium salt of α-sulfo fatty acid methyl esters (α-SFMeNa) and disodium α-sulfo fatty acids (α-SFNa2) by sulfonation of methyl esters of fatty acids (FMe) with sulfur trioxide (SO3) was investigated.
Methyl propionate and methyl stearate were sulfonated by dropwise addition of SO3 in carbon tetrachloride at -20°C, and the structure of the initial sulfonation product (I) was elucidated by the NMR analysis. The product (I) was FMe-SO3 adduct which was stable at -20°C, but when it was warmed to room temperature another SO3 attacked at α-hydrogen to give α-sulfonated FMe-SO3 adduct. This intermediate was neutralized with NaOH to give α-SFNa2 of which detergency and solubility were extremely poor. However, this intermediate was heated to 60°C and then neutralized to give α-SFMeNa which was known as an anionic surfactant with excellent biodegradability and detergency.
In a continuous thin film sulfonator, the higher the molar ratio of SO3 to FMe and the higher the reaction temperature were, the higher conversion ratio was obtained. However, the rate of the elimination reaction of the intermediate to α-SFMe was so slow that no more than 84% conversion ratio was obtained. Consequently, to gain the highest yields, the post reactor in which the inter-mediate was kept for a longer period at higher temperatures was necessary.
