Markovnikov Addition to Alkenes. A Different View from Core-Electron Spectroscopy and Theory.

Carbon 1s ionization energies for ethene, propene and 2-methylpropene have been measured in the gas phase at high resolution using synchrotron radiation and analysed by means of ab initio calculations. For the first time, resolution is good enough to assign ionization energies to the inequivalent carbons in propene and 2-methylpropene. A linear correlation is found between the ionization energies and activation energies for addition of the electrophiles HF, HCl, HBr and HI to these molecules. The correlation shows that both reactivity and regiospecificity are quantitatively related to core-ionization energies. Theoretical analysis of the core-ionization energies shows that the differences between ionization energies for the doubly bonded carbons are due to the charge distribution in the unionized molecule. Theoretical analysis of the transition state for addition of HCl to propene and 2-methylpropene indicates that a significant portion of the difference between Markovnikov and anti-Markovnikov addition is also due to the charge distribution in the initial state and not to different ability of the molecules to delocalize the added charge in the transition state. The increase in reactivity with the number of methyl groups is also strongly influenced by the initial-state charge distribution.