Symmetrical Methyl Group Transfers in the Gas-Phase

The high ionisation potential of the CHj radical makes methyl group transfers undergo the S 2 mechanism 

This class of reactions has been extensively studied, both experimentally and using ab initio methods and is one of the archetypal reactions of physical organic chemistry. Table 11.2 shows that the gas-phase internal barriers (Aft/) of symmetrical methyl group transfers are strongly dependent on the nature of the reactants. Additionally, the rates of the asymmetric reactions may change by ten orders of magnitude with the polarity of the solvent [6], 

It is convenient to start considering the study of methyl transfers with the reactions in the gas phase, which are free from solvent effects. However, there is a price to be paid for this simplification. In the gas phase these reactions proceed through the formation of a precursor ion-dipole complex in the absence of a barrier, followed by the actual methyl transfers and the formation of a successor ion-dipole complex, and the subsequent separation of the final products [9,10]. 

This traditional view explains the inversion of configuration observed experimentally in methyl transfers. However, it gives no leads to the motifs of the internal barriers reported in Table 11.2. Moreover, this view does not capture the driving force for the reaction how is it possible that the C-X bond breaks if all the electrons have bonding or non-bonding character  

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