Electrocyclic ring opening, electronic structure

Later, the structural chemists determined the structure of 1,3-butadiene (the bond lengths Cl—C2 and C2—C3 are 1.467 and 1.349 A, respectively, which can be compared with the corresponding values of fraw -2-butene (1.508 and 1.347 A), respectively). Another significant aspect of dienes is the Diels-Alder reaction, the reaction between a diene and an olefin with electron-withdrawing substituents to give a six-membered ring. The reaction is designated as 4 - - 2 cycloaddition since the diene has fonr carbon atoms while the olefin, a dienophile, may represent a two-carbon nnit. The mechanism of this useful reaction was not clear until 1964, when Woodward and Hoffmann proposed the so-called Woodward-Hoffmann rule. This proposal has opened a wide world of electrocyclic reactions in which the symmetry of orbitals plays an important role. 

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