The Origin of Alkyl Substituent Effects on the Reactivity and Thermodynamic Stability of Alkenes. A Theoretical Study on Alkene Radical Cations

Fully optimized STO‐3G SCF MO calculations were conducted on a series of methyl substituted ethylene radical cations. The stabilizing effect of a methyl group is found to be 14 kcal/mol using ethylene as a reference. Analysis of the orbital make‐up of the radical cations and their calculated geometry suggests that hyperconjugative 1‐, 2‐, and 3‐electron interactions between the alkyl groups and the π‐system is largely responsible for the stabilization. Analysis of hyperconjugation in propene suggests that 2‐electron stabilization outweighs 4‐electron destabilization energetically but that effects of the 2‐electron stabilization are manifested in a low‐lying MO (Φ₁, Fig. 6).