Mechanisms of Water Catalysed Reactions

Water is unique among liquids for its ability for tetrahedral coordination with four neighbouring molecules. Water catalyzed reactions (or spontaneous reactions) are pH independent hydrolytic processes that involve the transfer of one or several protons to or from water molecules in the transition state (TS) of the rate determining step. Indeed, in these reactions one water molecule acts as a nucleophile while one or several act as general bases, and typically show a) high negative values of entropy of activation (∆S ≠), and b) high kinetic solvent isotope effect (KSIE). In these reactions water is solvent, nucleophile and catalyst. The hydrolyses of alkyl halides, alkyl and aryl sulphonates, and derivatives of carboxylic acids ( esters, amides) have been extensively studied. The reaction with derivatives of saturated carbons is quite different than the hydrolyses of esters and amides. At least two water molecules must be strongly bound at the TS to produce a high KSIE, restricting the number of possible positions of these molecules. The number of molecules involved in the proton transfer ( two molecules per proton) can be determined by proton inventory. Polymolecular mechanisms have been found for these reactions with 3-5 molecules involved in the TS. Hydrophobicity and hydrophilicity of the substrate is determinant of the mechanism. Supramolecular Catalysis Induced by Polysaccharide