The Woodward-Hoffman (Hoffman) rule is a principle proposed by American chemists Roald Hoffman and Robert Woodward to describe the stereochemistry of synchronous reactions .
The main idea was this: orbital symmetry is maintained in synchronous reactions. That is, the reaction proceeds easily, if there is a correspondence between the characteristics of the orbital symmetry of molecular orbitals , if there is no such correspondence, then the reaction proceeds difficult. The rule applies to electrocyclization, cycloaddition reactions (including cheletropic transformations) and sigmatropic rearrangements .
For this theory, Hoffman received the Nobel Prize in Chemistry in 1981 (together with Kenichi Fukui ). Woodward died two years earlier and was therefore not on the list of nominees (this could be his second Nobel Prize).
Content
The principle of conservation of orbital symmetry
In the general case, the absence or presence of correspondence to orbital symmetry cannot be the only and final reason for the possibility or impossibility of a reaction. Symmetry is discontinuous, it can arise and disappear, be present or absent. The relationship between chemical phenomena and symmetry is not strictly expressed. For example, a weak fluctuation (say, the substitution of molecular fragments - the H atom for methyl CH 3 ) violates the general symmetry of the molecular system, but does not fundamentally change the reaction mechanism. An essential condition for the forbiddenness of the reaction is the presence in the transition state of at least one level that is not binding and is located in energy significantly higher than the other levels. In the transition state, higher energy levels can arise as a result of the intersection (which actually occurs) of the orbital energies. High energy levels are absent if each binding orbital of the final molecules comes from the connecting orbital of the original molecules. If some binding orbital of the final molecule does not come from some binding orbital of the original molecule, then it is formed with the participation of the loosening orbital of the original molecule. Moreover, the correlation between the binding and loosening orbitals depends on the presence or absence of general symmetry. If the initial correlation is broken, the level has high energy and is in a transition state.
Thus, it seems that the most realistic point of view is that the reaction proceeds with the preservation of orbital symmetry.
The principle of conservation of orbital symmetry facilitates the understanding and interpretation of reaction mechanisms. The rules of orbital symmetry prescribe the predominant course of reactions in which the filled orbitals of the reacting molecules and the orbitals of the final molecules completely correlate with each other. These rules reveal the reasons for the existence of an energy barrier to reactions, explain the coordinated (new bonds form with the destruction of old ones) and inconsistent (new bonds arise after the old ones break, and the system goes through a biradial state) reaction mechanisms. In accordance with these rules, a coordination mechanism is possible only when the initial and final states of the system are correlated.
Electrocyclic reactions
Using the Woodward-Hoffman rules, one can explain the stereospecificity of electrocyclic reactions proceeding under the influence of heat (thermally) or radiation ( photochemically ). In the original wording [1] , published in 1965 , the rules were as follows:
- In non-cyclic systems containing 4 n electrons, the symmetry of the highest occupied molecular orbital ( HOMO ) is such that the binding interaction between the ends of the chain should include overlapping between half-orbitals located on opposite sides of the plane of symmetry. This is possible only in the so-called. conrotational process :
- In non-cyclic systems containing 4 n + 2 electrons, the binding interaction of the orbitals between the ends of the chain requires the overlapping of half-orbitals located on one side of the plane of symmetry. This is implemented in the so-called. disrotator process :
- In photochemical reactions, an electron located on the HOMO of the reacting compound goes into an excited state. This leads to the opposite symmetry of the boundary orbitals and, accordingly, to the opposite stereospecificity of reactions
Reactions proceeding according to these requirements are called resolved by symmetry . Opposite reactions are forbidden by symmetry and require much more energy to proceed or do not go at all.
Literature
- Korolkov D.V., Skorobogatov G.A. Theoretical chemistry: Textbook. 2nd ed., Revised. and add. - St. Petersburg: Publishing House of St. Petersburg University, 2005. - 653 p. - ISBN 978-5-288-03639-2
- Woodworth R., Hoffman R. Preservation of orbital symmetry. - M .: Mir, 1971.
- Gilchrist T., Storr R. Organic reactions and orbital symmetry. - M., 1976.
Notes
- β Stereochemistry of Electrocyclic Reactions RB Woodward, Roald Hoffmann J. Am. Chem. Soc. , 1965 , 87 , 395β397. DOI : 10.1021 / ja01080a054