ja068120f_si_001.pdf (52.95 kB)
On the Theory of Organic Catalysis “on Water”
journal contribution
posted on 2007-05-02, 00:00 authored by Yousung Jung, R. A. MarcusA molecular origin of the striking rate increase observed in a reaction on water is studied
theoretically. A key aspect of the on-water rate phenomenon is the chemistry between water and reactants
that occurs at an oil−water phase boundary. In particular, the structure of water at the oil−water interface
of an oil emulsion, in which approximately one in every four interfacial water molecules has a free (“dangling”)
OH group that protrudes into the organic phase, plays a key role in catalyzing reactions via the formation
of hydrogen bonds. Catalysis is expected when these OH's form stronger hydrogen bonds with the transition
state than with the reactants. In experiments more than a 5 orders of magnitude enhancement in rate
constant was found in a chosen reaction. The structural arrangement at the “oil−water” interface is in
contrast to the structure of water molecules around a small hydrophobic solute in homogeneous solution,
where the water molecules are tangentially oriented. The latter implies that a breaking of an existing
hydrogen-bond network in homogeneous solution is needed in order to permit a catalytic effect of hydrogen
bonds, but not for the on-water reaction. Thereby, the reaction in homogeneous aqueous solution is
intrinsically slower than the surface reaction, as observed experimentally. The proposed mechanism of
rate acceleration is discussed in light of other on-water reactions that showed smaller accelerations in
rates. To interpret the results in different media, a method is given for comparing the rate constants of
different rate processes, homogeneous, neat and on-water, all of which have different units, by introducing
models that reduce them to the same units. The observed deuterium kinetic isotope effect is discussed
briefly, and some experiments are suggested that can test the present interpretation and increase our
understanding of the on-water catalysis.