An ab initio (HF/3-21+G*) direct dynamics quasiclassical trajectory study was performed for the Cl- + CH3Cl S(N)2 reaction at a reagent relative translational energy of 100 kcal/mol. Initial conditions for the trajectories were averaged over the orientation of CH3Cl and the reaction dynamics studied versus collision impact parameter. The trajectories reacted by a backside attack mechanism and reaction by frontside attack was not observed. The calculated backside reaction cross reaction is 0.22-0.40 Å2 and approximately two to four times larger than the experimental value (J. Phys. Chem. A 1997, 101, 5969). The absence of reaction by frontside attack was investigated by initiating trajectories at the C(s) transition state for the frontside attack mechanism. These trajectories formed Cl- + CH3Cl reactants with a large vibrational energy and low relative translational energy, which suggests extensive CH3Cl vibrational excitation is needed to access the frontside reaction pathway.