Pure and Applied Chemistry

Abstract: This article emphasizes two underappreciated aspects of hyperconjugation in hydrocarbons, two-way hyperconjugation and hyperconjugation in tight spaces. Nonplanar polyenes [e.g., cyclooctatetraene (D_2d), biphenyl (D₂), styrene (C₁)], the nonplanar rotational transition states (TSs) of planar polyenes (e.g., perpendicular 1,3-butadiene), as well as the larger nonplanar Hückel or Möbius annulenes, are stabilized by effective σ-electron delocalization (involving either the C–C or C–H bonds) via two-way hyperconjugation. The collective consequence of two-way hyperconjugation in molecules can be nearly as stabilizing as π-conjugation effects in planar polyenes. Reexamination of the σ- vs. π-bond strength of ethylene results in surprising counterintuitive insights. Strained rings and cages (e.g., cyclopropane and tetrahedrane derivatives, the cubyl cation, etc.) can foster unexpectedly large hyperconjugation stabilizations due to their highly deformed ring angles. The thermochemical stabilities of these species rely on a fine balance between their opposing destabilizing geometrical features and stabilizing hyperconjugative effects in tight spaces (adjustable via substituent effects). We hope to help dispel chemists’ prejudice in viewing hyperconjugation as merely a “mild” effect with unimportant consequences for interpreting the structures and energies of molecules.