Woodward-Hoffman rule of electrocyclic reactions

The photochemical dimerization of unsaturated hydrocarbons such as olefins and aromatics, cycloaddition reactions including the addition of 02 ( A ) to form endoperoxides and photochemical Diels-Alders reaction can be rationalized by the Woodward-Hoffman Rule. The rule is based on the principle that the symmetry of the reactants must be conserved in the products. From the analysis of the orbital and state symmetries of the initial and final state, a state correlation diagram can be set up which immediately helps to make predictions regarding the feasibility of the reaction. If a reaction is not allowed by the rule for the conservation of symmetry, it may not occur even if thermodynamically allowed. 

In these cycloaddition reactions which may be intermolecular, or intramolecular, a r-bond is converted into a c-bond. The reverse occurs when ring opening takes place. The closure or opening involves the movement of electrons and atoms but no atoms are gained or lost. Such transformations have been called electrocvclic reactions and may occur thermally or photoche.nically as governed by symmetry considerations. 

If we analyze the case of hexatriene to cyclohexadiene conversion, the situation is just the reverse. The thermal reaction should be disrotatory and the photochemical reaction conrotatory. The butadiene belongs to (4ri)n system and hexadiene to (4n +2)rc system, and a generalization of the systems may be attempted. 

In the case of conrotatory mode, the symmetry is preserved with respeo to C2 axis of rotation. On 180° rotation along this axis, F goes to H. and H2 to H, and the new configuration is indistinguishable from the original. An orbital symmetric with respect to rotation is called a and antisymmetric as b. On the other hand, in the case of disrotatory moot-the elements of symmetry are described with respect to a mirror plane. Tilt symmetry and antisymmetry of an orbital with respect to a mirror plant of reflection is denoted by a and a respectively (Section 2.9). The natun of each MO of cyclobutene with respect to these two operations is shov. n in the Table 8.4 for cyclobutene and butadiene. 

Axis of rotation conrotatory rrode Symmetry operation C 

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