Linear Free Energy Relationships in Kinetic Analyses: Applications of Quantum Chemistry

The role of linear free energy relationships and quantum chemistry in kinetic analyses is discussed and applied to a representative sequence of molecular pathways important in hydrocracking chemistry. Experimental data on adsorption, hydrogenation, cracking, isomerization, and coking for various aromatic, heteroatom-containing aromatics, and alkylaromatics were compiled from the literature and correlated with a set of reactivity indices which describe important structural, electronic, and/or energetic features of the reactants, products, or intermediates. All reactivity indices were computed from serniempirical quantum chemical calculations. The equilibria of adsorption were correlated with the proton affinity. Hydrogenation kinetics were related to the atomic site containing the highest π-electron density. Cracking kinetics were explained by the correlation with the heat of hydride transfer. Isomerization appeared dependent upon the heat of carbocation formation. Coke formation was related to the proton affinity. These LFERs provided a concise organizational summary of the chemistry of hydrocracking. Their potential in the prediction of the kinetics of unstudied systems is also discussed.