Molecular orbitals Woodward-Hoffman rules

For many reasons, including the Woodward-Hoffman rules that describe the likelihood of reaction based on arguments about the shapes of orbitals, it is desirable to be able to visualize molecular orbitals. 

If the reverse back reaction is prevented or is forbidden by other considerations, the energy remains stored in the photoproducts. Some simple photorearrangement reactions which are governed by Woodward-Hoffman rules have been found useful. These rules provide the stereochemical course of photochemical rearrangement based on symmetry properties of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the molecule (Section 8.6). A reaction which is photochemically allowed may be thermally forbidden. Front the principle of microscopic reversibility, the same will be true for the reverse reaction also. Thermally forbidden back reaction will produce. ble - photoproducts. Such electrocyclic rearrangements are given in . ..ure... 

Cycloadditions are controlled by orbital symmetry (Woodward-Hoffman rules) and can take place only if the symmetry of all reactant molecular orbitals is the same as the symmetry of the product molecular orbitals. Thus, an analysis of all reactant and product orbitals is required. A useful simplification is to consider only the frontier molecular orbitals. These orbitals are the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO). The orbital symmetry must be such that bonding overlap of the terminal lobes can occur with suprafacial geometry that is, both new bonds are formed using the same face of the diene. 

Woodward-Hoffman rules The stereochemistry of cicctrocyclic proce.sses. based on the symmetry of molecular tt orbitals. 

This approximation forming, for instance, the basis of the orbital symmetry conservation rules suggested by Woodward and Hoffman [525] gives a rather simple picture of the molecular electronic structure variation under inelastic collisions and chemical reactions as weU as a simple interpretation of the relative efficiency of different elementary processes. However, it has to be kept in mind that the one-electron approximation is rather rough. 

Over the years, different approaches have been developed to reveal chemical bonds. Covalent bonds are intuitively represented using conventional Lewis stractures [19]. Molecular Orbital (MO) theory has been veiy useful and successfiil for the theoretical analysis of chemical reactions and chemical reactivity. The frontier orbital theory [20] and the orbital symmetry rules of Woodward and Hoffman [21] are paradigmatic examples of the possibilities of quantum chemistry within the MO theory. 

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