Proton acceptor site selectivity in the proton transfer from heptane radical cations to decane molecules in γ-irradiated heptane/decane/1-chloroheptane crystals at 77 K

Abstract

A study is made of the yield and isomeric composition of chlorodecanes formed by γ-irradiation of heptane/decane/1-chloroheptane crystals at 77 K and subsequent warming. It is observed that in such systems 2-chlorodecane is predominantly formed with smaller yields of the other chlorodecane isomers, in analogy with observations of γ-irradiated CCl₃F/decane. The selective formation of 2-chlorodecane is attributed to proton transfer from heptane radical cations to decane molecules, (n-C₇H₁₆˙⁺ + n-C₁₀H₂₂ → 1-C₇H₁₅˙ + n-C₁₀H₂₃⁺), followed by neutralization of the so-formed protonated decanes by chloride ions upon warming. The results confirm that, when different secondary C–H bonds in neutral n-alkane molecules are directly accessible to the planar chain-end C–H bonds in n-alkane radical cations from which proton donation takes place, proton transfer occurs preferentially to the penultimate position (intrinsic acceptor site selectivity). Comparison of the isomeric composition of the chlorodecanes in heptane/decane/1-chloroheptane and CCl₃F/decane, on the other hand, clearly points to structurally-determined acceptor site selectivity with respect to the inner C–H bonds of decane in the heptane system.