Editing Conrotatory and disrotatory (section)

{{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
|}