Methyl Substitution Destabilizes Alkyl Radicals

We have quantum chemically investigated how methyl substituents affect the stability of alkyl radicals Me_mH_3−mC⋅ and the corresponding Me_mH_3−mC−X 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, the Me_mH_3−mC⋅ 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 Me_mH_3−mC−H molecule even more.