Proton transfer from alkane radical cations to alkane molecules: selectivity with respect to the site of proton donation and proton acceptance.
Information on the donor and acceptor site selectivity in the proton transfer from n-alkane radical cations to n-alkane molecules is gathered from gamma-irradiated frozen CCl(3)F/n-alkane solutions (symmetric transfer) and mixed n-alkane crystals (asymmetric transfer) by EPR spectroscopy at 77 K and gas chromatographic analysis after melting. The proton-donor site is related very strictly to the structure of the semi-occupied molecular orbital of the parent radical cation, with proton transfer taking place from those C-H bonds that carry appreciable unpaired-electron and positive-charge density. Proton acceptance is restricted to C-H bonds at secondary carbon atoms (no proton transfer to C-C bonds nor to C-H bonds at primary carbon atoms), with a preference for the penultimate position and equal (but considerably lower) transfer to the interior sites. In mixed n-alkane crystals, additional selectivity with respect to the site of proton acceptance results from structural factors in combination with the donor site selectivity (structurally determined acceptor site selectivity).
Department of Chemistry, K.U.Leuven, Celestijnenlaan 200-F, B-3001 Leuven, Belgium.
Jan
J
Ceulemans