Editing Chemical reaction (section)

===Other organic reaction mechanisms===
[[File:Cope Rearrangement Scheme.png|left|thumb|The Cope rearrangement of 3-methyl-1,5-hexadiene]]
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| image1= Diels Alder Mechanismus.svg
|caption1=Mechanism of a Diels-Alder reaction
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|caption2=Orbital overlap in a Diels-Alder reaction}}

In a [[rearrangement reaction]], the carbon skeleton of a [[molecule]] is rearranged to give a [[structural isomer]] of the original molecule. These include [[Sigmatropic reaction|hydride shift]] reactions such as the [[Wagner-Meerwein rearrangement]], where a [[hydrogen]], [[alkyl]] or [[aryl]] group migrates from one carbon to a neighboring carbon. Most rearrangements are associated with the breaking and formation of new carbon-carbon bonds. Other examples are [[sigmatropic reaction]] such as the [[Cope rearrangement]].<ref>{{cite book | author = Fox, Marye Anne | author2 = Whitesell, James K. | url = https://books.google.com/books?id=xx_uIP5LgO8C&pg=PA699 | title = Organic chemistry | edition = Third | publisher = [[Jones & Bartlett Learning|Jones & Bartlett]] | year = 2004 | isbn = 978-0-7637-2197-8 | page = 699}}</ref>

Cyclic rearrangements include [[cycloaddition]]s and, more generally, [[pericyclic reaction]]s, wherein two or more double bond-containing molecules form a cyclic molecule. An important example of cycloaddition reaction is the [[Diels–Alder reaction]] (the so-called [4+2] cycloaddition) between a conjugated [[diene]] and a substituted [[alkene]] to form a substituted [[cyclohexene]] system.<ref>{{Cite journal| volume = 460| pages = 98–122| year = 1928 | doi = 10.1002/jlac.19284600106| journal = Justus Liebig's Annalen der Chemie| title = Synthesen in der hydroaromatischen Reihe| first2 = K.| last2 = Alder| last1 = Diels| first1 = O.}}</ref>

Whether a certain cycloaddition would proceed depends on the electronic orbitals of the participating species, as only orbitals with the same sign of [[wave function]] will overlap and interact constructively to form new bonds. Cycloaddition is usually assisted by light or heat. These perturbations result in a different arrangement of electrons in the excited state of the involved molecules and therefore in different effects. For example, the [4+2] Diels-Alder reactions can be assisted by heat whereas the [2+2] cycloaddition is selectively induced by light.<ref>[[#Bruckner|Brückner]], pp. 637–647</ref> Because of the orbital character, the potential for developing [[stereochemistry|stereoisomeric]] products upon cycloaddition is limited, as described by the [[Woodward–Hoffmann rules]].<ref>{{Cite journal | doi = 10.1021/ja01080a054| title = Stereochemistry of Electrocyclic Reactions| journal = Journal of the American Chemical Society| volume = 87| issue = 2| pages = 395–397| year = 1965| last1 = Woodward | first1 = R.B.| last2 = Hoffmann | first2 = R. | bibcode = 1965JAChS..87..395W}}</ref>