Kinetic solvent effects in aqueous solution

Water is unique among solvents because it has only an exceptionally small molecular volume but also the capacity to form a 3-D H-bond network with a dominant tetrahedral coordination. Kinetic solvent effects in water and in highly aqueous binary solvent mixtures respond to these properties. Organic reactivity can be best analyzed using transition state 

This difference in Gibbs energy is strongly influenced by hydration effects. For a bimolecular reaction, dehydration plays an important role since water has to be removed from the hydration shells of both reaction partners to allow sufficiently close contact for overlap of those molecular orbitals responsible for formation of the AC. Jorgensen has shown in his Monte Carlo computer simulations of the Sn2 process 

Following a tradition pioneered by Ingold, solvent effects are often intuitively assigned to dominating effects on the AC. Indeed, usually the AC has an increased polarity compared to the IS. But for reactions in water, in many cases the specific medium effect of water most strongly influences the IS. Therefore it is important to make a distinction between hydration effects on the IS and AC. Let us see how this can be done.  

Monte Carlo simulation with approximately 500 solvent molecules 

Unknown From activity From kinetic coefficient measurements measurements 

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