Editing Conrotatory and disrotatory

{{Short description|Classification of electrocyclic reactions by how end groups are rotated}}
{{No footnotes|date=April 2020}}

In [[organic chemistry]], an [[electrocyclic reaction]] can either be classified as '''conrotatory''' or '''disrotatory''' based on the rotation at each end of the [[molecule]]. In conrotatory mode, both [[atomic orbital]]s of the [[end group]]s turn in the same direction (such as both atomic orbitals rotating clockwise or counter-clockwise).  In disrotatory mode, the atomic orbitals of the end groups turn in opposite directions (one atomic orbital turns clockwise and the other counter-clockwise). The [[cis–trans isomerism|cis/trans geometry]] of the final product is directly decided by the difference between conrotation and disrotation.

Determining whether a particular reaction is conrotatory or disrotatory can be accomplished by examining the [[molecular orbital]]s of each molecule and through a set of rules. Only two pieces of information are required to determine conrotation or disrotation using the set of rules: how many [[Pi bond|electrons]] are in the [[Conjugated system|pi-system]] and whether the reaction is induced [[Thermochemistry|by heat]] or [[Photochemistry|by light]]. This set of rules can also be derived from an analysis of the molecular orbitals for predicting the [[stereochemistry]] of electrocyclic reactions.

{| class="wikitable" border="1"
|-
! System
! Thermal
! Photochemical
|-
| "4n" electrons
| Conrotatory
| Disrotatory
|-
| "4n + 2" electrons
| Disrotatory
| Conrotatory
|}

==Example of a photochemical reaction==
Analysis of a [[Electrocyclic_reaction#Excited_state_electrocyclizations|photochemical electrocyclic reaction]] involves the [[HOMO]], the [[LUMO]], and correlations diagrams.

An electron is promoted into the LUMO changing the frontier molecular orbital involved in the reaction.

==Example of a thermal reaction==
Suppose that trans-cis-trans-2,4,6-octatriene is converted to {{chem name|dimethylcyclohexadiene}} under thermal conditions.  Since the substrate octatriene is a "4n + 2" molecule, the [[Woodward–Hoffmann rules]] predict that the reaction happens in a disrotatory mechanism.

Since thermal electrocyclic reactions occur in the HOMO, it is first necessary to draw the appropriate [[molecular orbital diagram|molecular orbitals]]. Next, the new carbon-carbon bond is formed by taking two of the p-orbitals and rotating them 90 degrees (see diagram). Since the new bond requires constructive overlap, the orbitals must be rotated in a certain way. Performing a disrotation will cause the two black lobes to overlap, forming a new bond. Therefore, the reaction with octatriene happens through a disrotatory mechanism.

In contrast, if a conrotation had been performed then one white lobe would overlap with one black lobe.  This would have caused destructive interference and no new carbon-carbon bond would have been formed.

In addition, the cis/trans geometry of the product can also be determined. When the p-orbitals were rotated inwards it also caused the two methyl groups to rotate upwards. Since both methyls are pointing "up", then the product is {{chem name|'''cis'''-dimethylcyclohexadiene}}.

[[File:Disrotatory electrophilic reaction molecular orbitals.svg|center|Disrotatory ring closing reaction]]

==References==
* Carey, Francis A.; Sundberg, Richard J.; (1984). Advanced Organic Chemistry Part A Structure and Mechanisms (2nd ed.). New York N.Y.: Plenum Press. {{ISBN|0-306-41198-9}}.
* March Jerry; (1985). Advanced Organic Chemistry reactions, mechanisms and structure (3rd ed.). New York: John Wiley & Sons, inc. {{ISBN|0-471-85472-7}}

[[Category:Physical organic chemistry]]