The presence of a solvent interacting with a system throughout its evolution from reactants to products brings about qualitative changes from the corresponding gas-phase reaction. There are changes in both the reaction rate and the dynamics. The energetic effects due to the solvent reflect the electronic reorganization that takes place as the system transverses the reaction path.The SN2 ion–molecule reaction, shown in Figure 11.1 for the generic X- + CH3X exchange reaction, provides an example in which the charge delocalization in the transition-state region causes qualitative changes in the energy profile along the reaction coordinate when the reaction is in the presence of a polar solvent. As becomes clear in this chapter, even the reaction coordinate itself is not exactly the same in solution as it is in the gas phase, because the solvent adaptation to the changing system must also be considered.

A solute molecule at room temperature undergoes of the order of 1013 collisions per second with solvent molecules. The solvent can therefore hinder the large-amplitude motions that often accompany chemical transformations (e.g., as in a twist isomerization, Figure 11.14). The cage effect, where the solvent hinders the separation of the products, Figure 1.8, or the approach of the reactants, was one of the first examples of the role of the solvent. The cage effect remains a major difference between gas-phase and solution dynamics. There can also be dynamical effects.