Theoretical study of ion pair S(N)2 reactions: Ethyl vs methyl reactivities and extension to higher alkyls

Identity S(N)2 reactions of methyl fluoride and chloride with fluoride and chloride ions are compared computationally with the ion pair displacement reactions of lithium fluoride and chloride and of sodium fluoride. The results with the RHF MP2 and B3LYP methods and the 6-31+G* basis set essentially confirm some previous results obtained by RHF and MPn methods. The ion pair reactions proceed via an initial dipole-dipole complex and a cyclic transition structure with highly bent X-C-X bonds that resemble a collection of two cations and two anions. Extension of the calculations to the corresponding ethyl systems gives lower ion pail barriers than methyl, completely different from the corresponding ionic barriers. The bending away of the halogens in the ion pair transition structures reduces steric effects. Analysis of charge distributions shows that in the ionic transition structures the nearby halogens electrostatically inhibit polarization and delocalization of charge. The nonlinear X-C-X bond angle in the ion pair transition structures coupled with the electrostatic influence of the cation permits greater charge polarization and delocalization within the organic moiety and allows the greater stability of ethyl cation compared to methyl to manifest itself, n-Propyl, isopropyl, and neopentyl chlorides have lower barriers than ethyl in reaction with lithium chloride but isopropyl and neopentyl chlorides have higher barriers than ethyl toward chloride ion.