Effects of anionic micelles on intramolecular general base-catalysed aminolysis of phenyl and methyl salicylates

Abstract

The effects of micelles of sodium dodecyl sulphate (SDS) on aminolysis of ionized phenyl salicylate (PS^–) and methyl salicylate (MS^–) have been studied at 35 °C. An increase in the total concentration of SDS ([SDS]_T) from 0.0 to 0.2 mol dm^–3 Results in a decrease in the observed nucleophilic second-order rate constants (k_n) by a factor of ca. 3 for the reactions of PS^– with propylamine and 1-aminopropan-2-ol. At high [SDS]_T, plots of observed pseudo-first-order rate constants (k_obs)versus total propylamine concentration ([Am]_T) appear to exhibit smaller slopes at [Am]_T < 0.01 mol dm^–3 compared with those at [Am]_T > 0.01 mol dm^–3, while for 1-aminopropan-2-ol such deviations from linearity at [Am]_T < 0.01 mol dm^–1 could not be detected. These observations are attributed to the higher hydrophilicity of 1-aminopropan-2-ol compared with that of propylamine. The values of k_n for hydrazinolysis of MS^– are decreased ca. 1.7-fold and those for hydroxylaminolysis MS^– are increased ca. 2-fold with an increase in [SDS]_T from 0.0 to 0.2 mol dm^–3. The values of k_n for methylaminolysis of MS^– are independent of [SDS]_T within the limits 0.0-0.2 mol dm^–3. Dimethylamine did not show any detectable nucleophilic reactivity toward MS^– in the presence of 0.03 mol dm^–3 SDS. This shows that the presence of SDS perhaps does not change the nucleophilic reaction mechanism of aminolysis of salicylate esters. The observed results of aminolysis of PS^– and MS^– are rationalized in the light of the proposal of a porous cluster micellar structure.