THEORETICAL-STUDIES OF ELIMINATION-REACTIONS .3. GAS-PHASE REACTIONS OF F- WITH (CH3)2CHCL AND CH3CH2CH2CL - THE EFFECT OF METHYL SUBSTITUENTS

The effect of methyl substitution on E2 and S(N)2 mechanisms was evaluated by applying high-level ab initio calculations to the gas-phase reactions of F- with (CH3)2CHCI and CH3CH2CH2CI. E2 (anti and syn) as well as S(N)2 pathways were investigated, and transition states were located at the 6-31+G* level. The nature of all stationary points was confirmed with analytical frequencies. Energy comparisons were made at the MP2/6-31+G**//HF/6-31+G* level corrected for zero-point vibrations (scaled by 0.9). As expected, the addition of a methyl group at the alpha-carbon increases the S(N)2 barrier (by 2.2 kcal/mol); however, in the proper conformation, a methyl group at the beta-carbon reduces the barrier (by 1.7 kcal/mol). Methyl groups at either carbon stabilize the E2 transition states by about 2-3 kcal/mol. Both systems have a strong stereochemical preference (approximately 13 kcal/mol) for anti rather than syn eliminations. The E2(anti) transition states are periplanar, and their geometries suggest a synchronous E2 elimination. The E2(syn) reactions are more E1cb-like and involve syn clinal rather than syn periplanar transition states. The FH(beta)-C(beta)-C(alpha)-Cl dihedral is approximately 35-degrees. In the syn transition states, the barrier to rotation around the C(alpha)-C(beta) bond is small and the transition-state energy varies little for dihedral angles between 0 and 60-degrees (1-3 kcal/mol). A review of past theoretical work indicates that syn clinal rather than syn periplanar conformations may be generally preferred for gas-phase syn eliminations. A comparison of the energetics of the S(N)2 and E2 reactions predicts that elimination will dominate in the propyl systems.