Lj. Saethre et al., MARKOVNIKOV ADDITION TO ALKENES - A DIFFERENT VIEW FROM CORE-ELECTRONSPECTROSCOPY AND THEORY, Perkin transactions. 2, (4), 1997, pp. 749-755
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.