Matthias Bickelhaupt’s group (VU Amsterdam) turns yet another generally accepted ‘fact’ in (physical) organic chemistry on its head: methyl radicals are not stabilised by more substitutions, but rather destabilised. They provide the proof in Angewandte Chemie.

Previously, we wrote about paradigm shifts in the bond length and bond strength of C-H bonds, but Bickelhaupt’s group has not been idle. They are now bringing evidence against a well-known rule of thumb in organic chemistry, namely that the more substituted a methyl radical is (in the form MemH3-mC∙, with m = 0-3), the more stable it will be. This would consequently be the reason for the weakening of the C-H bond of the MemH3-mCH alkane at higher substitution. Eva Blokker and colleagues at the Vrije Universiteit Amsterdam provide evidence that the first claim is incorrect, while the second claim holds.

Bickelhaupt had been walking around with this idea for some time, PhD student Eva Blokker tells us. ‘He always had the idea that methyl substituents on carbon radicals would also lead to steric repulsion of the radical, which would therefore make the radical less stable.’ The textbook explanation that a more substituted radical would be more stable seemed to be wrong in Bickelhaupt’s eyes.