Introduction to Reaction Mechanisms
Quick Notes
- A reaction mechanism is a step-by-step description of how a chemical reaction occurs at the molecular level.
- Each step is called an elementary reaction.
- The mechanism includes:
- Reactants – present from the start.
- Products – formed at the end.
- Intermediates – formed in one step and used up in another.
- Catalysts – added at the start, regenerated by the end.
- The sum of all elementary steps must match the overall balanced equation.
Full Notes
What Is a Reaction Mechanism?
Many chemical reactions occur through a sequence of elementary steps:
Each step involves the breaking or forming of a small number of bonds. The overall reaction is the sum of all elementary steps that occur (see Elementary Reactions). How the steps link together is referred to as the reaction mechanism.
Key point: A reaction mechanism describes the steps a chemical reaction follows as reactants turn into products. Together, the steps explain how the reaction proceeds and help us understand which molecules interact, in what order, and how quickly.
Components of a Reaction Mechanism
- Reactants – The species present at the beginning of the reaction.
- Products – The substances formed at the end of the mechanism.
- Intermediates – Molecules that are formed in one step and then consumed in a later step. They do not appear in the overall equation.
Example If A reacts in the following way, A → B → C, then B is an intermediate.
- Catalysts – Substances that appear in one step and are regenerated by (or in) the final step. They do not appear in the overall, balanced reaction equation.
Be sure you understand the difference between intermediates and catalysts:
• Intermediates are formed and used up.
• Catalysts are used and reformed.
Writing and Validating a Mechanism
- All elementary steps must add up to the overall balanced chemical equation.
- The mechanism should be consistent with experimental observations, especially the rate law and any detected intermediates.
- Intermediates can often be inferred from experiments, and detecting one provides strong evidence supporting a proposed mechanism.
Summary
A reaction mechanism shows how a chemical reaction progresses through a series of elementary steps. It includes reactants, products, intermediates (temporary species), and catalysts (helpers that are not consumed). The mechanism must add up to the overall balanced reaction and should be consistent with experimental evidence, including observed rate laws and detected intermediates.