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{{short description|Chemical reaction which forms a cyclic molecule}}
In [[organic chemistry]], a '''cycloaddition''' is a [[chemical reaction]] in which "two or more [[Unsaturated hydrocarbon|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 [[Multiplicity (chemistry)#Molecules|bond multiplicity]]". The resulting reaction is a [[cyclization]] reaction. Many but not all cycloadditions are [[Concerted reaction|concerted]] and thus [[pericyclic]].<ref name="goldbook">{{Citation|title=cycloaddition|url=https://goldbook.iupac.org/html/C/C01496.html|work=IUPAC Compendium of Chemical Terminology|year=2009|publisher=IUPAC|doi=10.1351/goldbook.C01496|isbn=978-0-9678550-9-7|access-date=2018-10-13|doi-access=free|url-access=subscription}}</ref> Nonconcerted cycloadditions are not pericyclic.<ref>{{Citation|title=pericyclic reaction|url=https://goldbook.iupac.org/html/P/P04491.html|work=IUPAC Compendium of Chemical Terminology|year=2009|publisher=IUPAC|doi=10.1351/goldbook.P04491|isbn=978-0-9678550-9-7|access-date=2018-10-13|doi-access=free|url-access=subscription}}</ref> 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]]: {{nowrap|(''i'' + ''j'' + …)}} where the variables are the numbers of linear atoms in each reactant. The product is a cycle of size {{nowrap|(''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 />
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