A History of the Double-Bond Rule

Abstract

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.

Accessing cationic tetrylene-nickel(0) systems featuring donor–acceptor E–Ni triple bonds (E = Ge, Sn)

“…1 The past two decades have seen this interest migrate to the heavier group 14 elements, Si–Pb: where once it was thought multiple bonds involving these elements were not viable ( viz. the double bond rule), we now know that this is not the case, 2 with an ever-growing number of multiply-bonded species involving these heavier tetryl elements. 3 In 1996, the first compound featuring an E–M triple bond (E = Si–Pb, M = transition metal) was reported by the group of Power, in the molybdenum–germylyne complex A , 4 followed soon after by analogous chromium- and tungsten-germylyne complexes B and C (Fig.…”

Geometrically Compelled Disilene with λ4-Coordinate SiIIAtoms

“…The ability of heavy group 14 elements to form multiple bonds was not realized until 1973 when Lappert reported the first stable compound with a SnSn bond . This breakthrough contradicted the paradigm of the so-called “double-bond rule” and heralded the existence of many new classes of isolable low-coordinate group 14 element species, foremost in 1981 the synthesis of the first stable disilene (SiSi) and silene (SiC) by West and Brook, respectively. In contrast to ethylene, which adopts a planar D 2h geometry, disilene has as a trans-bent equilibrium structure in which the SiSi bond has partly π and charge transfer type character (Chart A) .…”