Multistep Mechanisms

The overall reaction process often corresponds to a series of two or more elementary steps, which must always add up to give the overall balanced chemical equation. For example, in the following elimination reaction (a type of reaction typically discussed in introductory organic chemistry courses):

C₄H₉Br + H₂O → C₄H₈ + H₃O⁺ + Br^–

This reaction process proceeds in two elementary steps and would therefore be called a two-step mechanism. In the first step, bromide leaves from the starting material to give the cation C₄H₉⁺. In the second step, C₄H₉⁺ reacts with water to generate the product C₄H₈ and H₃O⁺.

1. C₄H₉Br ⇌ C₄H₉⁺ + Br^–                   (slow)

2. C₄H₉⁺ + H₂O ⇌ C₄H₈ + H₃O⁺

Net: C₄H₉Br + H₂O → C₄H₈ + H₃O⁺ + Br^–

The cation C₄H₉⁺ is called an intermediate, since it does not appear in the overall balanced equation and is generated in one elementary step but used up in a subsequent step. A potential energy diagram for this multistep reaction can be drawn as shown in Figure 1.1 “Multistep Reaction Potential Energy Diagram.” Notice each step has its own activation energy, and transition state (or activated complex), which is the highest-energy transitional point in the elementary step. Transition states are very unstable (high energy) as bonds are in the process of breaking or forming, and therefore transition states cannot be isolated. Intermediates are more stable than transition states and can sometimes be isolated and characterized by certain techniques.

Figure 1.1. Multistep Reaction Potential Energy Diagram

Multistep reaction potential energy diagram showing the intermediate