Single-ion enthalpies of transfer as a scale of nucleophilic reactivity towards ethyl iodide in acetonitrile

Abstract

Logarithmic rates for nucleophilic substitution towards ethyl iodide in acetonitrile correlate well with the specific interaction enthalpies for the relevant nucleophile, ΔH_t,SI^{AN → MeOH}(Nu^–). According to the correlation, the bromide ion reaction falls into the imidide ion reaction series, while the chloride ion reaction falls into the carboxylate ion reaction series. The partial desolvation accompanying activation is crucial in determining the reactivity behaviour of the nucleophile anions. Semi-empirical quantum mechanical calculations (MNDO/PM3 and MNDO/PM3/COSMO) give reasonable estimates for the enthalpies of activation as well as for reaction enthalpies in acetonitrile. The relative location of the transition state, estimated on the basis of empirical analysis, is in accord with the variation of electronic charges for the exposed atoms, which has been evaluated through semi-empirical quantum mechanical procedures.