Proton Transfer from Alkane Radical Cations to Alkanes

Abstract

Alkane radical cations are very strong Brønsted acids capable of protonating alkanes. Radiation-chemical studies have proven highly successful for the investigation of such proton transfer from alkane radical cations to alkane molecules, using n-alkane nanoparticles embedded in cryogenic CCl 3 F matrices for the study of symmetric proton transfer (RH .+ + RH fi R . + RH 2 + ) and mixed n-alkane crystals for the study of asymmetric proton transfer (R I H .+ + R II H fi R I . + R II H 2 + ). The mechanism of the radiolytic processes involved is discussed in considerable detail. Selectivity with respect to the site of both proton donation and proton acceptance has been studied, using EPR spectroscopy at 77 K for the study of proton donation and gas chromatographic analysis after melting for the study of proton acceptance. The experiments described allow one to conclude that the protondonor 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-hole density. With respect to the site of proton acceptance, it is observed that chemical transformation due to protonation of n-alkanes by alkane radical cations in the systems studied is restricted to C-H bonds at secondary carbon atoms (no chemical transformation resulting from proton transfer to C-C bonds or to C-H bonds at primary carbon atoms). It is argued that the absence of chemical transformation due to C-C protonation has a complex origin in which thermochemical and structural (cage) effects are involved as well as the intrinsic stability of the carbonium ions, but that the absence with respect to protonation of C-H bonds at primary carbon atoms has (in all likelihood) a purely thermochemical origin. As to protonation of n-alkanes at secondary C-H bonds, extensive evidence is presented supporting a marked preference for the penultimate position, with considerably lower (and mutually equal) transfer to the interior sites (intrinsic acceptor-site selectivity). 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).