What is Conrotatory and disrotatory?
These terms describe two classes of an [[electrocyclic reaction]] (a type of [[organic chemistry|organic]] [[chemical reaction]]s). In a '''conrotatory''' mode, the [[substituent]]s located at the ends of a [[conjugated system|conjugated]] [[covalent bond|double bond]] system move in the same ([[clockwise]] or [[anticlockwise|counterclockwise]]) direction during ring opening or ring closure. In a '''disrotatory''' mode, they move in opposite directions. An example is the [[conversion]] of [[Geometric isomerism|trans]]-cis-trans-2,4,6-octatriene to [[Geometric isomerism|cis]]-dimethylcyclohexadiene (top of figure). The [[molecular orbital|orbital]] mechanics of the reaction needs a disrotatory mode. The orbital symmetry of the octatriene's [[HOMO/LUMO|highest occupied molecular orbital (HOMO)]] requires for the end pi orbitals to move in opposite directions to form the correct [[symmetry]] found in the [[sigma bond]]. [[Image:disrotatory.png|center|Disrotatory ring closing reaction]] [[heat|Thermal]] rearrangements of all conjugated systems containing 4n + 2 [[pi electron]] are [[stereospecific]]. This is based on preservation of [[orbital symmetry]] in the [[HOMO/LUMO|highest occupied molecular orbital]]. Systems containing 4n pi electrons show the opposite [[conrotatory]] mode. This is also true for rearrangements of 4n + 2 pi (where n is a [[whole number]]) electrons driven by light ([[photoinduced]]). Photoinduced rearrangements of 4n pi electron systems (where the number of electrons is divisible by 4) follow the disrotatory rule. The [[Woodward–Hoffmann rules]] summarize the above different reactions. The following picture also shows the difference between conrotatory and disrotatory reactions: [[File:Torquo.png|center|thumb|600px|Conrotatory and disrotatory modes of rotation each showing two possible directions of rotation that result in pairs of [[enantiomer]]s for a generic hexatriene system.]] ==References== * Carey, Francis A.; Sundberg, Richard J.; (1984). Advanced Organic Chemistry Part A S