MARKOVNIKOV ADDITION TO ALKENES - A DIFFERENT VIEW FROM CORE-ELECTRONSPECTROSCOPY AND THEORY

Carbon Is ionization energies for ethene, propene and 2-methylpropene have been measured in the gas phase at high resolution using synchrotr on 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 cor relation is found between the ionization energies and activation energ ies for addition of the electrophiles HF, HCl, HBr and HI to these mol ecules. The correlation shows that both reactivity and regiospecificit y are quantitatively related to core-ionization energies. Theoretical analysis of the core-ionization energies shows that the differences be tween ionization energies for the doubly bonded carbons are due to the charge distribution in the unionized molecule. Theoretical analysis o f the transition state for addition of HCl to propene and 2-methylprop ene indicates that a significant portion of the difference between Mar kovnikov and anti-Markovnikov addition is also due to the charge distr ibution in the initial state and not to different ability of the molec ules to delocalize the added charge in the transition state. The incre ase in reactivity with the number of methyl groups is also strongly in fluenced by the initial-state charge distribution.