Alkyl groups around a carbon atom destabilise it, making the resulting carbocation relatively more stable. This fact is unjustly overlooked, write Dutch theoretical chemists in Chemical Communications. At the same time, they point out the danger of using isodesmic reactions.

Carbocations are reactive intermediates in organic chemistry that you see frequently in fundamental synthetic reactions. These reactive particles are generally formed by breaking a C-X bond, for example carbon-halogen compounds such as R3C-I or R3C-Cl. Until now, it was thought that this C-X bond became weaker if you added more and more alkyl groups to the carbon atom, because this would increase the stability of the carbocation. ‘That is certainly not incorrect,’ says Thomas Hansen of the Vrije Universiteit Amsterdam. ‘It just doesn’t give the complete picture, there is more behind it.’ Hansen published the complete picture in Chemical Communications with Pascal Vermeeren, Matthias Bickelhaupt and Trevor A. Hamlin, all three affiliated with the same university. Recently, the article even made the cover.

Hansen and his colleagues show that the more important effect takes place in the starting molecule: by adding more and more alkyl groups (in this case methyls) to R3C-X, it becomes less stable. That is what drives the stabilisation of the carbocation, which the group believes has been completely overlooked so far. ‘The hyperconjugation between the cation and the substituents is also important, but it is not the main contributor,’ Hansen stresses.