electrocyclic reaction In organic chemistry, an electrocyclic reaction is a type of pericyclic rearrangement where the net result is one pi bond being converted into one sigma bond or vice versa. These reactions are usually categorized by the following criteria: * Reac ...

can either be classified as conrotatory or disrotatory based on the rotation at each end of the molecule. In conrotatory mode, both atomic orbitals of the end groups 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 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 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.

Example of a photochemical reaction

Analysis of a photochemical electrocyclic reaction involves the

HOMO ''Homo'' () is the genus that emerged in the (otherwise extinct) genus '' Australopithecus'' that encompasses the extant species ''Homo sapiens'' ( modern humans), plus several extinct species classified as either ancestral to or closely relat ...

, the

LUMO In chemistry, HOMO and LUMO are types of molecular orbitals. The acronyms stand for ''highest occupied molecular orbital'' and ''lowest unoccupied molecular orbital'', respectively. HOMO and LUMO are sometimes collectively called the ''frontie ...

, 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 dimethylcyclohexadiene under thermal conditions. Since the substrate octatriene is a "4n + 2" molecule, the

Woodward–Hoffmann rules The Woodward–Hoffmann rules (or the pericyclic selection rules), devised by Robert Burns Woodward and Roald Hoffmann, are a set of rules used to rationalize or predict certain aspects of the stereochemistry and activation energy of pericyclic rea ...

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 orbitals In chemistry, a molecular orbital is a mathematical function describing the location and wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of findin ...

. 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 cis-dimethylcyclohexadiene.

References

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