THEORETICAL-STUDIES OF ELIMINATION-REACTIONS .1. REACTIONS OF F- AND PH2- WITH CH3CH2CL - COMPETITION BETWEEN SN2 AND E2 MECHANISMS FOR 1ST-ROW AND 2ND-ROW NUCLEOPHILES

In this study the gas-phase reactions of F- and PH2- (proton affinities 371.5 and 370.9 kcal mol, respectively) with CH3CH2Cl were characterized with ab initio methods. For each nucleophile, S(N)2,E2(anti), and E2(syn) reaction paths were investigated and transition-state structures were located. At the MP4SDQ/6-31(+)G**//HF/6-31(+)G* level of theory, activation barriers were calculated. Analytical frequencies were computed for each transition state and minimum, and all energies are corrected for zero-point vibrations (scaled by 0.9). With F- as the nucleophile, negative activation barriers (-6.7 and -5.7 kcal/mol, respectively) were found for the S(N)2 and E2(anti) reactions, indicating that substitution and elimination should be competitive. A positive activation barrier of +7.0 kcal/mol was found for the E2(syn) reaction, indicating a strong preference for an antiperiplanar transition state. With PH2-, the S(N)2 reaction has a small activation barrier (+5.8 kcal/mol), but the E2(anti) and E2(syn) have very large activation barriers (+17.4 and +25.8 kcal/mol). Although F- and PH2- have nearly the same thermodynamic basicities, their abilities to induce elimination reactions are very different. The theoretical results are consistent with recent gas-phase studies which indicate that first-row nucleophiles are well-suited for both S(N)2 and E2 reactions whereas second-row nucleophiles are more limited to S(N)2 reactions. Atomic electron populations and critical point densities were computed with Bader's method and are used to develop a model that explains the nucleophilic preferences in terms of bond polarities.