The mechanism of the formation of sodium salt of α-sulfo fatty acid methyl esters (α-SFMeNa) and disodium α-sulfo fatty acids (α-SFNa
2) by sulfonation of methyl esters of fatty acids (FMe) with sulfur trioxide (SO
3) was investigated.
Methyl propionate and methyl stearate were sulfonated by dropwise addition of SO
3 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-SO
3 adduct which was stable at -20°C, but when it was warmed to room temperature another SO
3 attacked at α-hydrogen to give α-sulfonated FMe-SO
3 adduct. This intermediate was neutralized with NaOH to give α-SFNa
2 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 SO
3 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.
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