Molecular Design, Catalysis, and Reaction Mechanisms
Mechanism as an explanatory framework
What a mechanism should explain
A molecular reaction mechanism proposes a sequence of elementary steps that accounts for observed kinetics, selectivity, and intermediates. Mechanistic inference blends theory, spectroscopy, isotopic labeling, and computation, often under uncertainty.
Mechanistic humility
Multiple mechanisms can fit the same macroscopic rate law; orthogonal evidence is often required to distinguish them.
Catalysis across environments
Catalyst classes
| Catalyst type | Strengths | Typical challenges |
|---|---|---|
| Homogeneous | Molecular precision, tunable ligands | Separation and stability |
| Heterogeneous | Robustness, scalability | Active-site heterogeneity |
| Enzymatic | Exceptional selectivity | Complex conformational dynamics |
What is a common role of a catalyst in a reaction?
Catalysts accelerate rates primarily by lowering the free-energy barrier along a reaction pathway.
Correct answer: Lower the activation free energy
Why can enzymes be viewed as molecular machines?
Function arises from coupled conformational and chemical changes.
Correct answer: They use coordinated structure, dynamics, and active-site interactions to promote specific chemical transformations
Selectivity and design principles
Controlling outcome
Chemoselectivity, regioselectivity, and enantioselectivity emerge from competing pathways on nearby free-energy surfaces. Design strategies include ligand engineering, substrate preorganization, steric gating, and electrostatic tuning.
Which factor most directly distinguishes enantioselective catalysis from achiral catalysis?
Enantioselective catalysts create a chiral environment that lowers the barrier for one enantiomeric outcome.
Correct answer: A chiral environment that favors one enantiomeric pathway
What is one reason reaction rates can be poor even when products are thermodynamically favored?
Thermodynamics and kinetics are distinct: favorable products can still be inaccessible on practical timescales.
Correct answer: The kinetic barrier may be high
Frontier questions in catalysis
Open problems