A molecular orbital approach to a comprehensive cracking mechanism for linear long chain alkanes in heterogeneous acid catalytic conditions through carbenium ion β-cleavage

An extensive MINDO/3 theoretical study of the elementary steps of n-heptane cracking has been performed. A mechanism involving a carbenium ion is proposed and then a number of possibilities for further evolution of the different carbenium ions are considered. The energy surfaces for hydride transfer, β-cleavage processes either direct or concerted with 1,2-hydrogen shifts or concerted with a cyclic rearrangement, are explored. The energy barriers so obtained range over ca. 50 kcal mol–1 and are insensitive to product size. Consequently, the final product distribution cannot be explained in terms of the β-scission of linear n-heptane as the rate-limiting step. The β-scission energy barrier for some branched seven-carbon carbenium ions have been calculated to be ca. 30 kcal mol–1, an intermediate value between linear β-cracking and branching rearrangement transition energies. A mechanism is proposed that implies a fast achievement of equilibrium between different branched carbocations after carbenium ion generation by hydride transfer and then irreversible β-cracking.