Concerted reaction

A chemical reaction in which bond-making and bond-breaking occur in a single elementary step, without a discrete intermediate.

Transition state: A high-energy molecular configuration at a local maximum along a reaction pathway. A concerted reaction passes through one transition state rather than a sequence of intermediate states.
Diels–Alder reaction: A cycloaddition between a conjugated diene and an alkene or alkyne that forms a six-membered ring. This classic pericyclic reaction forms two carbon–carbon bonds in a concerted step.
Stepwise reaction: A reaction that proceeds through two or more elementary steps separated by one or more intermediates. Unlike a concerted pathway, it has an intermediate between successive bond-reorganization steps.
Stereospecificity: A property in which different stereoisomeric reactants produce different stereoisomeric products by a defined mechanism. Concerted geometry often preserves a direct relationship between reactant stereochemistry and product stereochemistry.
Elementary reaction: A single molecular event represented by one step in a reaction mechanism. Concerted reactions are elementary reactions with multiple bond changes in the same event.
Potential energy surface: A function describing a molecule’s potential energy as its nuclear coordinates vary. Its energy profile distinguishes a concerted pathway from stepwise routes with intermediate minima.
Claisen rearrangement: A thermal rearrangement of an allyl vinyl ether into a carbonyl compound through a six-membered transition state. It illustrates concerted reorganization of several bonds in a predictable cyclic geometry.
Carbocation rearrangement: A molecular rearrangement in which a carbocation changes its carbon skeleton through shifts of atoms or groups. Its discrete carbocation intermediate contrasts with concerted rearrangements.
Kinetic isotope effect: A change in reaction rate caused by replacing an atom with one of its isotopes. Isotope effects can reveal whether a bond to the substituted atom changes in the rate-limiting step.
Pericyclic reaction: A reaction involving a cyclic redistribution of bonding electrons through a concerted transition state. Many textbook examples of concerted reactions belong to this mechanistic family.

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