Ortho Effect. V:Rates of Alkaline Hydrolysis of Ethyl Benzoates in Aqueous Dimethyl Sulfoxide.

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In order to confirm the previous observation that the ortho effect in the alkaline hydrolysis of substituted ethyl benzoates is not affected by the type of solvent, the rates of hydrolysis have further been measured in a mixture of water and dimethyl sulf oxide (DMSO), a typical dipolar aprotic solvent. The rates of hydrolysis of non-substituted benzoate in 65 vol.% DMSO-water, 85 vol . % DMSO-water, 95 vol. % DMSO-water, and 99 vol.% DMSO-water were found to be 56, 267, 1, 840, and 16, 000 times, respectively, as much as that observed in 85 wt % ethanol-water. The Hammett's p value increased with the concentration of DMSO and thus the value in 95.vol % DMSO-water was 3.1 in contrast to 1.33 in 3 wt. % ethanolwater. Despite such marked solvent effects, the ortho effect still remains almost constant. The accelerating effect characteristic to DMSO has been discussed based on the reaction rates of non-substituted benzoate, p values, and the ortho effects observed in mixtures of DMSO-water as well as ethanol-water . In 99 vol.% DMSO-water, the ortho effect induced by the o-methyl substituent essentially disppeared probably due to desolvation.
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