5 years ago

SN2 Reaction Rate Enhancement by β-Cyclodextrin at the Liquid/Liquid Interface

SN2 Reaction Rate Enhancement by β-Cyclodextrin at the Liquid/Liquid Interface
Ilan Benjamin, John J. Karnes
An inverse phase transfer catalyst typically enhances the rate of biphasic reaction by bringing the water-insoluble reactant from the organic to the aqueous phase. We use the empirical valence bond (EVB) approach to obtain reaction free energy profiles for a model SN2 reaction inside the β-cyclodextrin (β-CD) cavity at the water/1-bromooctane interface and in bulk water to show that a significant rate enhancement is taking place at the liquid/liquid interface rather than in the bulk. By examining several solvent–solute structural and energetic properties, we demonstrate that the rate enhancement when the reaction takes place inside the cavity at the interface is primarily due to limited accessibility of interfacial water molecules, which results in destabilization of the reactants. Greater accessibility of water molecules when the catalyst is in the bulk stabilizes the reactants and does not lead to rate enhancement despite the significant hydrophobicity of the cavity’s interior.

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b05091

DOI: 10.1021/acs.jpcc.7b05091

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