Gutmann's Donor Numbers Correctly Assess the Effect of the Solvent on the Kinetics of SNAr Reactions in Ionic Liquids

Abstract

We report an experimental study on the effect of solvents on the model SNAr reaction between 1-chloro-2,4dinitrobenzene and morpholine in a series of pure ionic liquids (IL). A significant catalytic effect is observed with reference to the same reaction run in water, acetonitrile, and other conventional solvents. The series of IL considered include the anions, NTf2-, DCN-, SCN-, CF3SO3-, PF6-, and FAP(-) with the series of cations 1-butyl-3-methyl-imidazolium ([BMIM]+), 1-ethyl-3-methyl-imidazolium ([ EMIM]+), 1-butyl2,3- dimethyl-imidazolium ([BM2(I)M](+)), and 1-butyl-1-methyl-pyrrolidinium ([BMPyr](+)). The observed solvent effects can be attributed to an "anion effect". The anion effect appears related to the anion size (polarizability) and their hydrogenbonding (HB) abilities to the substrate. These results have been confirmed by performing a comparison of the rate constants with Gutmann's donicity numbers (DNs). The good correlation between rate constants and DN emphasizes the major role of charge transfer from the anion to the substrate.