Mixed Aqueous Solvent Effects on Kinetics and Mechanisms of Organic Reactions
Water and mixtures of water with organic solvents are commonly used as solvents for a large variety of organic reactions. Over the last two decades, the mechanisms of many of these processes have been investigated in some detail, usually by kinetic techniques. Comparison of these results with those obtained for the same reactions in nonaqueous media have impressively illustrated that water and highly aqueous mixed solvents display remarkable and often amazing solvent effects on the rates and thermodynamic activation parameters of nucleophilic displacement reactions, acid—base catalyzed processes, and many other important reactions in organic chemistry. An interesting example of a large solvent rate effect is provided by the uncatalyzed decarboxylation of 6-nitrobenzisoxazole-3carboxylate (1) studied by Kemp and Paul(476) and also discussed by Jencks.(446) Upon changing the reaction medium from hexamethylphosphoramide decreased by a factor of ~108. In ethanol the rate retardation is about 106. Apparently, delocalization of the negative charge in the transition state for decarboxylation (2) is highly unfavorable in hydrogen bonding solvents and especially in water. On the other hand, numerous reactions are known that involve either the creation or localization of charge in the transition state of the rate-limiting step and which are markedly accelerated if an aprotic solvent medium is replaced by water.
KeywordsTransition State Solvent Effect Organic Reaction Solvent Composition Typically Aqueous
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