The concept of a hierarchy of unimolecular reactions in a homologous series. Prediction of the unimolecular chemistry of some saturated carbenium ions

Abstract

The unimolecular chemistry of the saturated cations C_n H_2n+1(n= 2–12) is examined on the basis that such ions, when produced by electron impact, undergo (in slow reactions) decomposition over the lowest available energy surfaces. There is no evidence for any isolated states which could, in principle, undergo slow reactions (log₁₀K= 4–6) from high energy states. Elimination of neutral molecules (H₂, CH₄) with σ-bond formation is found for the lower homologues (C₂H₅⁺, C₃H₇⁺, C₄H₉⁺). In higher homologues (C₅H₁₁⁺–C₁₂H₂₅⁺), only olefin eliminations are observed in slow reactions. With only the assumptions that (a) the various possible olefin eliminations compete one against the other and (b) the reverse reactions (combination of an olefin and a carbenium ion) have zero, or near zero, activation energies, the unimolecular chemistry of the C₅-C₁₂ saturated carbenium ions can be either accounted for or predicted.