Reaction Mechanisms
A reaction mechanism is a detailed step-by-step description of how reactants transform into products in a chemical reaction. It explains the individual elementary steps, including bond formation and breaking, atom rearrangement, and transition states.
Key Components of Reaction Mechanisms
1. Elementary Steps
These are individual molecular events within a mechanism, such as molecular collisions, intermediate formation, or bond breaking.
Types of Elementary Steps and Their Rate Laws:
- Unimolecular: One reactant molecule (e.g., decomposition).
- Bimolecular: Two reactant molecules collide.
- Termolecular: Three reactant molecules (rare due to low probability).
2. Intermediates
Species formed during the reaction that do not appear in the final products. Intermediates are usually unstable and short-lived before converting to products.
3. Transition States
High-energy states occurring during the conversion of reactants to products. Represented as peaks on an energy diagram, they mark the maximum energy point along the reaction pathway.
4. Rate-Determining Step (RDS)
The slowest elementary step in a mechanism that limits the overall reaction rate. The rate law of the overall reaction is often derived from the RDS.
Example of a Reaction Mechanism
Consider the reaction:
A + B → C
Possible mechanism:
A + B → AB(fast) — formation of intermediateABAB → C(slow) — intermediate decomposes to product
The overall reaction is the sum of these steps. The slow Step 2 is the rate-determining step, governing the reaction rate.
Importance of Reaction Mechanisms
- Predicting Reaction Rates: Understand how concentration, temperature, or catalysts affect rate.
- Designing New Reactions: Enables creation of new synthetic routes by controlling reaction steps.
Additional Theories
- Collision Theory: Reactions occur when molecules collide with proper orientation and energy.
- Transition State Theory: Focuses on the activated complex formed during the transition state and energy barriers.
