Bond Strengths: The Importance of Hyperconjugation

Abstract

Gronert (J. Org. Chem.2006, 71, 1209) has challenged the importance of hyperconjugation in determining C−H bond dissociation enthalpies (BDEs) in alkanes. Electron paramaganetic resonance spectra of H₃CCH₂^•, (H₃C)₂CH^•, and (H₃C)₃C^• show significant positive spin on their β-H₃C groups' hydrogens. A 55%/45% partitioning of these spins between hyperconjugation and spin polarization mechanisms linearly correlates with the C−H BDEs in methane, ethane, propane, isobutane and propene. Hyperconjugation is an important factor determining alkane C−H BDEs.