Methyl Substitution Destabilizes Alkyl Radicals

We have quantum chemically investigated how methyl substituents affect the stability of alkyl radicals MemH3-mC center dot and the corresponding MemH3-mC center dot bonds (X = H, CH3, OH; m = 0 - 3) using density functional theory at M06-2X/TZ2P. The state-of-the-art in physical organic chemistry is that alkyl radicals are stabilized upon an increase in their degree of substitution from methyl< primary< secondary< tertiary, and that this is the underlying cause for the decrease in C H bond strength along this series. Here, we provide evidence that falsifies this model and show that, on the contrary, themH(3-m)C(center dot) radical is destabilized with increasing substitution. The reason that the corresponding C-H bond nevertheless becomes weaker is that substitution destabilizes the sterically more congested MemH3-mC-H molecule even more.

Automated summary

This study investigates how methyl substituents affect the stability of alkyl radicals and provides evidence that contradicts the existing model. It is found that although the radicals become less stable with increasing substitution, the corresponding C-H bond weakens due to the increased steric congestion in the molecule.