Ionic Liquids: Additives for Manipulating the Nucleophilicity

Kinetic investigations of the SN2 reaction at the sulfur atom of p-toluenesulfonyl chloride with sodium azide in dried methanol in the presence of varying amounts of room temperature ionic liquids (RTILs), 1-butyl-3-methylimidazolium acetate ([C4C1im][CH3COO]), 1-butyl-3-methylimidazolium chloride ([C4C1im]Cl) and 1-butyl-3-methylimidazolium hexafluorophosphate ([C4C1im][PF6]), were carried out in order to explore and understand the impact of these additives on the rate of such reactions. The observed results indicate that the rate constant of the reaction increase appreciably with increases in the concentration of RTILs in RTIL–methanol binary solvent systems. The results were analyzed in light of a Kamlet–Taft model system, which established that the observed impact of RTILs can be attributed to the cumulative effects of increase in the β value (hydrogen bonding acceptor ability) which is expected to enhance the reactivity of p-toluenesulfonyl chloride as well as the nucleophilicity of the azide ion and decrease in the π* value (solvent dipolarity/polarizability) which is expected to enhance the reactivity of the azide ion. Of the three ionic liquids used in the presented studies, [C4C1im][CH3COO] was observed to be more effective in accelerating the rate constant; this we attribute to its comparatively stronger ability to increase the β value and decrease the π* value in the mixed solvent system.