Nucleophilic substitution in binary mixed solvents. Kinetics and transfer enthalpies of anions in the mixed solvents methanol + propylene carbonate and methanol+N-methyl-2-pyrrolidone

Abstract

Rate constants and activation enthalpies for the reaction of ethyl iodide with bromide ion have been determined in two solvent mixtures, i.e., methanol + propylene carbonate and methanol+N-methyl-2-pyrrolidone mixtures. Heats of solution have also been determined for ethyl iodide, tetra-n-butylammonium-bromide, -perchlorate and -tetra-n-butylborate in the two solvent mixtures. Judging from these transfer enthalpies, perchlorate ion seems to be a pertinent model compound for the activated complex of the reaction. Single ion transfer enthalpies have been evaluated on the basis of the (n-Bu)₄NB(n-Bu)₄ assumption in the two solvent mixtures. Transfer enthalpy against composition profiles for perchlorate ion are similar in the two solvent mixtures, whereas for bromide ion the corresponding profiles are quite different in the two solvent mixtures, i.e., a sharp minimum was observed for the profile in MeOH + PC mixtures. From these observations and from other evidence, it is concluded that bromide ion solvation involves both electrostatic interactions and specific interactions between solute and solvents. Model calculations performed on the basis of the above views reproduce the bromide ion profile in MeOH + PC mixtures well.