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 disp lacement reactions of lithium fluoride and chloride and of sodium fluo ride. The results with the RHF MP2 and B3LYP methods and the 6-31+G b asis set essentially confirm some previous results obtained by RHF and MPn methods. The ion pair reactions proceed via an initial dipole-dip ole complex and a cyclic transition structure with highly bent X-C-X b onds that resemble a collection of two cations and two anions. Extensi on of the calculations to the corresponding ethyl systems gives lower ion pail barriers than methyl, completely different from the correspon ding ionic barriers. The bending away of the halogens in the ion pair transition structures reduces steric effects. Analysis of charge distr ibutions shows that in the ionic transition structures the nearby halo gens electrostatically inhibit polarization and delocalization of char ge. The nonlinear X-C-X bond angle in the ion pair transition structur es coupled with the electrostatic influence of the cation permits grea ter charge polarization and delocalization within the organic moiety a nd allows the greater stability of ethyl cation compared to methyl to manifest itself, n-Propyl, isopropyl, and neopentyl chlorides have low er barriers than ethyl in reaction with lithium chloride but isopropyl and neopentyl chlorides have higher barriers than ethyl toward chlori de ion.