Reaction mechanism

A reaction mechanism is a stepwise account of the elementary reactions by which reactants become products, specifying bond changes and the movement of electrons.

Elementary reaction: A single molecular event that occurs in one step, with a defined molecularity and rate law. Mechanisms are assembled from elementary reactions rather than treated as single overall events.
Reaction kinetics: The study of reaction rates and how they depend on concentrations, temperature, and other conditions. Rate laws constrain which sequences of elementary steps can explain a reaction.
Organic reaction: A chemical transformation involving compounds built principally from carbon. Reaction mechanisms are especially detailed tools for explaining organic transformations.
Chemical equation: A symbolic representation of reactants, products, and their stoichiometric proportions in a chemical reaction. An equation gives overall stoichiometry but usually omits the elementary steps connecting reactants and products.
Curved arrow notation: A notation in organic chemistry that uses arrows to show the movement of electron pairs or single electrons. Curved arrows map the electron movements that connect one mechanistic step to the next.
Rate-determining step: A step in a reaction mechanism that strongly limits the overall reaction rate. Its barrier often controls the rate patterns used to test mechanisms.
Nucleophilic substitution: A reaction in which a nucleophile replaces a leaving group at an electrophilic center. Its distinct one-step and two-step pathways show how mechanisms predict different outcomes.
Reaction pathway: A sequence of states and transformations connecting reactants to products on a potential-energy surface. A pathway describes the route through molecular states; a mechanism specifies the chemical steps along it.
Reaction intermediate: A transient chemical species formed in one reaction step and consumed in a later step. Intermediates distinguish multistep mechanisms from single-step transformations.
Kinetic isotope effect: A change in reaction rate caused by replacing an atom with one of its isotopes. Isotope-dependent rates can reveal whether a bond is made or broken in a key step.

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