Theoretical Study of the Mechanism of Catalytic Alkylation of Adamantane with 2,2,4-Trimethylpentane Cracking Products

Abstract

Quantum-chemical calculations on the mechanism of catalytic alkylation of adamantane with isooctane cracking products using the density functional theory DFT B3LYP/6-31G* have been carried out. It has been shown that the initial stage of transformations is the interaction of AlCl₃ · HCl (as a model of an acid catalyst) with isooctane. At the first cracking stage, proton transfer from the catalyst to isooctane occurs (activation energy is calculated to be 24.64 kcal/mol) to give intermediate 1, which consists of three interacting subsystems: the cation (СН₃)₃С⁺; the anion \({\text{AlCl}}_{4}^{ - }\); and СН₃–СН(СН₃)₂, i.e., isobutane. The second stage is proton transfer from the carbocation (СН₃)₃С⁺ to form the olefin CH₂=C(CH₃)₂. The activation energy of this stage was calculated to be 7.85 kcal/mol. This is the final stage of isooctane cracking, yielding an olefin and an alkane smaller than the parent one. The mechanism of formation of the adamantyl cation has been considered, in which the cation adds the olefin without activation energy and the resulting complex can form either 1-isobutylenyladamantane (unsaturated byproduct of adamantane alkylation) via deprotonation by the catalyst anion or the final product 1-isobutyladmantane via interaction with another adamantane molecule. The latter can also be formed by the joint action of isobutane and the catalyst anion on the adamantyl cation, with the activation energy being 26.79 kcal/mol.