Chemical reactivities orbital symmetry conservation

The interpretation of chemical reactivity in terms of molecular orbital symmetry. The central principle is that orbital symmetry is conserved in concerted reactions. An orbital must retain a certain symmetry element (for example, a reflection plane) during the course of a molecular reorganization in concerted reactions. It should be emphasized that orbital-symmetry rules (also referred to as Woodward-Hoffmann rules) apply only to concerted reactions. The rules are very useful in characterizing which types of reactions are likely to occur under thermal or photochemical conditions. Examples of reactions governed by orbital symmetry restrictions include cycloaddition reactions and pericyclic reactions. 

In 1952, Fukui published his Frontier MO theorywhich went initially unnoticed. In 1965, Woodward and Hoffmann published their principle of conservation of orbital symmetry, and applied it to all pericyclic chemical reactions. The immense success of these rules" renewed interest in Fukui s approach and together formed a new MO-based framework of thought for chemical reactivity (called, e.g., giant steps forward in chemical theory in Morrison and Boyd, pp. 934, 939, 1201, and 1203). This success of MO theory dealt a severe blow to VB theory. In this area too, despite the early calculations of the Diels-Alder and 2-1-2 cycloaddition reactions by Evans,VB theory missed making an impact, in part at least because of its blind adherence to simple resonance theory. All the subsequent VB derivations of the rules (e.g., by Oosterhoff in Ref. 90) were after the fact and failed to reestablish the status of VB theory. 

The question about the transformations of reactants into products by the molecular reaction (direct way) and chain (complicated, multistage way) was considered in the monograph by N.N. Semenov On Some Problems of Chemical Kinetics and Reactivity. The new approach appeared because the quantum chemistry formulated the rule of conservation of orbital symmetry in chemical and photochemical reactions (Woodward-Hofinann rule). Let us consider this interesting aspect. 

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