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 CCl3F/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).