Reaction Mechanisms and Elementary Steps
A reaction mechanism is the sequence of elementary molecular steps by which reactants become products, and kinetics provides the tools to test proposed mechanisms against observed rates.
Definition
A reaction mechanism is a detailed, step-by-step account of the elementary reactions and intermediates that connect reactants to products, consistent with the experimentally observed rate law and stoichiometry.
Scope
This topic covers the construction and testing of reaction mechanisms: elementary steps and their molecularity, reactive intermediates, the rate-determining step, and the chaining of steps into overall reactions. It develops the approximations used to derive rate laws from mechanisms, including the steady-state approximation for reactive intermediates and the pre-equilibrium approximation, and applies them to chain reactions, the Lindemann-Hinshelwood scheme for unimolecular reactions, and enzyme and surface catalysis. The empirical rate law itself and the theory of single-step rate constants are covered elsewhere.
Core questions
- What distinguishes an elementary step from an overall reaction?
- How does the steady-state approximation yield a rate law from a proposed mechanism?
- How does the rate-determining step control the overall reaction rate?
- How are chain reactions and unimolecular decompositions explained mechanistically?
Key concepts
- Elementary step and molecularity
- Reactive intermediates
- Rate-determining step
- Steady-state and pre-equilibrium approximations
- Chain reactions and the Lindemann mechanism
Key theories
- Steady-state approximation
- When a reactive intermediate is consumed almost as fast as it is formed, its concentration can be taken as nearly constant, allowing its elimination from the kinetic equations to derive the overall rate law from the elementary steps.
- Lindemann-Hinshelwood mechanism for unimolecular reactions
- Apparently unimolecular reactions proceed by bimolecular collisional activation followed by unimolecular decomposition, explaining why their effective order falls from first toward second as pressure decreases.
Clinical relevance
Mechanistic understanding guides the rational design of catalysts and synthetic routes, the suppression of unwanted side reactions, the interpretation of combustion and atmospheric chemistry such as ozone destruction, and the analysis of enzyme catalysis and drug metabolism.
History
Bodenstein introduced the steady-state idea and the concept of chain reactions around 1913; Lindemann's 1922 proposal explained unimolecular kinetics, and the chain-reaction theory of Semenov and Hinshelwood in the 1920s and 1930s established the mechanistic analysis of branching and explosive reactions.
Key figures
- Frederick Lindemann
- Cyril Norman Hinshelwood
- Max Bodenstein
Related topics
Seminal works
- atkins2018
- laidler1987
Frequently asked questions
- Can experiments ever prove a reaction mechanism is correct?
- No. Kinetics can rule mechanisms out when they predict the wrong rate law, and consistency with rate data, intermediate detection, and isotope effects builds confidence, but a mechanism remains a model that can always be refined or replaced by better evidence.
- What is the difference between molecularity and order?
- Molecularity counts the number of species colliding in a single elementary step and is always a small whole number, whereas order is the empirical power of concentration in the overall rate law and can be fractional or zero because it reflects a whole multi-step mechanism.