Characterization of anion solvation in N-methylacetamide. Transfer enthalpies of anions and the reaction rates of ethyl iodide with bromide ion in N-methylacetamide–acetonitrile and N-methylacetamide–N,N-dimethylacetamide mixtures

Abstract

Single-ion transfer enthalpies have been determined in N-methylacetamide–acetonitrile and N-methylacetamide–N,N-dimethylacetamide mixtures for bromide, perchlorate, and tetra-n-butylborate ions on the basis of the tetra-n-butylammonium/tetra-n-butylborate assumption. From both empirical and theoretical treatments of these quantities it is concluded that N-methylacetamide interacts with these spherical and pseudo-spherical anions through the N—H dipole directed towards the anion, i.e. it behaves as a protic solvent. Transfer enthalpies for the transition-state anion have also been determined for the ethyl iodide plus bromide ion reaction in the two solvent mixtures. The presence and the magnitude of the specific interaction between the rod-like transition-state anion and the amide linkage, due to dipole–dipole association, have been evaluated on various grounds. The relative alignment of N-methylacetamide towards the anion seems to change as the reaction proceeds from reactants to a transition state.