Seminars Archive

Reactivity of model hydrocarbons at high-pressure

Abstract
Pressure is a thermodynamic parameter of paramount importance for chemical equilibria and chemical kinetics. In molecular substances the effect of increasing the pressure is to reduce the volume available for the system to such an extent that an increasingly repulsive part of the intermolecular potential is explored. At sufficiently high confining pressure the repulsive interactions are balanced and intramolecular and intermolecular forces are comparable in magnitude making the system thermodynamically unstable. The free energy can be minimized through a full reorganization of the chemical bonds connectivity with processes like ionization, polymerization, formation of atomic lattices, metallization and the like. This effect can be more pronounced (and can be induced even at moderate pressures) when unsaturated chemical bonds are present in the material. The system will react, to the pressure where electron instability is induced, by substituting the multiple bonds and the weak intermolecular bonds with a distribution of single bonds. The chemical reactivity at ultrahigh pressure, and specifically the kinetics and the reaction mechanisms, can differ drastically from those observed at ambient conditions. The geometrical confinement of the reacting molecules poses strong constraints and leads the molecules to react with precise steric arrangements so that a higher degree of selectivity can be expected. Pressure could be therefore exploited as an activation, initiation or regulatory tool of chemical reactions alternative to catalysts or other chemical and physical means. This would offer the non negligible advantage that high pressure reactions are mostly carried in the absence of solvents and occur quantitatively thus avoiding unwanted byproducts. In this seminar these aspects will be analyzed by examinating the reactivity of several model hydrocarbons at high pressure. The products, the kinetic and the selectivity of the reaction at high pressure will be reviewed in order to extract information on a tailor-made high pressure chemistry.