A simple rationale for lowered stabilities of branched and cross-conjugated polyenes

Abstract

The study is aimed at revealing the decisive factors for relative stabilities of acyclic p-electron systems of polyenes, the carbon backbones of which are of different type of branching. The systems are modeled as sets of N weakly interacting double (C=C) bonds. The relevant total p-electron energies are represented in the form of power series containing members of even orders with respect to the averaged resonance parameter of single (C-C) bonds. For distinct isomers of the same polyene, both zero-order energies and respective second-order corrections are shown to take uniform values. Relative stabilities of these isomers are then primarily determined by the fourth-order member of the series that, in turn, consists of two additive components of opposite signs, viz., of the stabilizing component expressible as a sum of transferable increments of individual triplets of linearly conjugated C=C bonds [i.e., of the three-membered conjugated paths (CPs)] and of the destabilizing component depending on overall adjacencies (connectivity) of C=C bonds. Lower stabilities of p-electron systems of branched and/or crossconjugated polyenes vs. the linear ones then follow from comparative analyses of the relevant fourth-order energies, and this destabilization is shown to originate either from (a) a reduced number of CPs or from (b) higher adjacencies of C=C bonds in the former isomers. An actual interplay of both factors (c) also is rather common. The three cases (a)-(c) are illustrated by specific examples. Graphical abstract

Genealogy of Conjugated Acyclic Polyenes

“…On the other hand, Gineityte’s discussion is too sophisticated and specific to be followed by a majority of chemists [ 11 ].…”

“…Fortunately, however, novel methods for synthesizing dendralenes (vide infra) and the related hydrocarbons have recently been discovered and several researchers have reconsidered the importance of the role of cross-conjugation in organic chemistry [ 7 , 8 , 9 , 10 , 11 , 12 ].…”

Perturbational non-canonical theory of molecular orbitals and its applications

“…(2.4.12) The expressions of Eqs. (2.4.8), (2.4.11) and (2.4.12) have been successfully applied in studies of relative stabilities of linear and branched isomers of polyenes [73], as well as of individual Kekulé valence structures of benzenoids [38] and phenylenes [61].…”

“…It is also assumed that our hydrocarbon contains no adjacent double bonds and no cycles having an odd number of carbon atoms. Evidently, this model primarily refers to acyclic polyenes [40,58,73,77]. Nevertheless, it has been applied recently also to individual Kekulé valence structures of related (poly)cyclic compounds [38,61].…”

“…2) The rebonding effect in conjugated hydrocarbons manifests itself as an interdependence between orders of the newly-formed C−C bonds and alterations in the orders of the "initial" C=C bonds. In particular, the (negative) second order correction to the order of the Ith C=C bond (determined by the element J (1) and C−C bonds [73]].…”