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The tautomeric polar systems recognized by Laar in 1886 contain an active atom that appeared to migrate from its original position. The tautomeric systems are of a general structural form and can be represented as X=Y-Z-A. Later workers recognized the same bond weakening effect in a variety of organic structures in which atom A is halogen, hydrogen, carbon, or nitrogen. Hermann Staudinger recognized the weakness of that bond, an allyl bond, in hydrocarbons and exploited the behavior for the preparation of isoprene from terpene hydrocarbons. In 1922 he formulated a generality, a rule, regarding the allyl bond reactivity He noted that natural rubber also decomposed to form isoprene and therefore concluded that natural rubber is an unsaturated hydrocarbon, that isoprene units in natural rubber represent weakly held allyl substituents, and that natural rubber is a macromolecular combination of isoprene units.
From his different experience as an industrial chemist, Otto Schmidt recognized the same bond weakening effect in hydrocarbons and in 1932 postulated the "Double-Bond Rule," stating that the presence of a double bond in a hydrocarbon has an alternating strengthening and weakening effect on single bonds throughout the molecule, diminishing with distance from the double bond. Schmidt not only understood the practical benefit of this rule, but he also offered an explanation for the Rule on theoretical grounds. Novel in its time, his theoretical explanation did not find popular acceptance, despite his considerable efforts to promote it in the literature. His concept of the Rule was supplanted by the new theory of resonance devised by Pauling and Wheland and by the implied notion of the stabilization of products by delocalization effects.
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