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A '''cycloaddition''' is a [[pericyclic]] [[chemical reaction]], in which "two or more unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity."<ref name=goldbook>{{cite web|last=International Union of Pure and Applied Chemistry (IUPAC)|title=Cycloaddition|url=http://goldbook.iupac.org/C01496.html|accessdate=26 February 2014}}</ref> The resulting reaction is a [[cyclization]] reaction. Many but not all cycloadditions are [[Concerted reaction|concerted]]. As a class of [[addition reaction]], cycloadditions permit carbon–carbon bond formation without the use of a [[nucleophile]] or [[electrophile]].
Cycloadditions can be described using two systems of notation. An older but still common notation is based on the size of linear arrangements of atoms in the reactants. It uses [[parentheses]]: (''i'' + ''j'' + …) where the variables are the numbers of linear atoms in each reactant. The product is a cycle of size (''i'' + ''j'' + …). In this system, the standard [[Diels-Alder reaction]] is a (4 + 2)cycloaddition, the [[1,3-dipolar cycloaddition]] is a (3 + 2)cycloaddition and [[cyclopropanation]] of a carbene with an alkene a (2 + 1)cycloaddition.<ref name=goldbook />
A more recent, IUPAC-preferred notation uses [[square brackets]] to indicate the number of ''electrons'', rather than carbon atoms, involved in the formation of the product. In the [''i'' + ''j'' + …] notation, the standard Diels-Alder reaction is a [4 + 2]cycloaddition, the 1,3-dipolar cycloaddition is [4 + 2].<ref name=goldbook />
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