Markovnikov addition to alkenes. A different view from core-electron spectroscopy and theory
Abstract
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.
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