Conrotatory and disrotatory

In organic chemistry, an electrocyclic reaction can either be classified as conrotatory or disrotatory based on the rotation at each end of the molecule.

Determining whether a particular reaction is conrotatory or disrotatory can be accomplished by examining the molecular orbitals 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 electrons are in the pi-system and whether the reaction is induced by heat or by light.

This set of rules can also be derived from an analysis of the molecular orbitals for predicting the stereochemistry of electrocyclic reactions.

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