Characteristics of the S_N2 Reaction

Now that we know how S_N2 reactions occur, we need to see how they can be used and what variables affect them. Some S_N2 reactions are fast, and some are slow; some take place in high yield and others in low yield. Understanding the factors involved can be of tremendous value. Let’s begin by recalling a few things about reaction rates in general.

The rate of a chemical reaction is determined by the activation energy ΔG^‡, the energy difference between reactant ground state and transition state. A change in reaction conditions can affect ΔG^‡ either by changing the reactant energy level or by changing the transition-state energy level. Lowering the reactant energy or raising the transition-state energy increases ΔG^‡ and decreases the reaction rate; raising the reactant energy or decreasing the transition state energy decreases ΔG^‡ and increases the reaction rate (Figure 1.1). We’ll see examples of all these effects as we look at S_N2 reaction variables.

Figure 1.1 The effects of changes in reactant and transition-state energy levels on reaction rate. (a) A higher reactant energy level (red curve) corresponds to a faster reaction (smaller ΔG^‡). (b) A higher transition-state energy level (red curve) corresponds to a slower reaction (larger ΔG^‡).