Ester hydrolysis factors affecting rate

A pseudo-first-order reaction can be defined as second-order reaction involving two reacting species that behave like a first-order reaction. This type of reaction can occur if one reactant is present in greater excess. Under these circumstances, even though there are two reactants in the solution, the reaction rate can be determined by the concentration of only one reactant. Hydroxyl ion-catalyzed ester hydrolysis is an example of a pseudo first-order reaction. For detailed information on kinetics of reactions, the equations involved, and factors affecting rates of reaction, the reader is advised to seek the available references (88,89) and Chapter 11 of this book. 

It should be noted that the inductive effect is not the only factor affecting the rate of hydrolysis. The substituent may also have a steric effect. For example, a bulky substituent may shield the ester from attack and lower the rate of hydrolysis. It is therefore necessary to separate out these two effects. This can be done by measuring hydrolysis rates under basic conditions and also under acidic conditions. Under basic conditions, steric and electronic factors are important, whereas under acidic conditions only steric factors are important. By comparing the rates, values for the electronic effect (ar), and for the steric effect (Es) (see below) can be determined. 

The factor 2.48 puts a on the same scale as Hammett s er, and the k0 values are rate constants for acid and base hydrolysis of acetic acid esters (i.e., R is a methyl group in the reference compound). Usually R is an ethyl or methyl group, but in many cases the rate constants do not depend on the nature of R. Equation 8 is based on the fact that acid hydrolysis rates of substituted benzoic acid esters are only slightly affected by the nature of the substituent, but acid hydrolysis rates of aliphatic esters are strongly affected by substituents. These effects were taken to be caused by steric factors thus log(/c//c0)acid defines s. It is reasonable to assume that steric factors affect base-catalyzed rates in the same way. Substituent effects on base hydrolysis of aliphatic compounds are composed of both polar and steric effects, and subtraction of the latter yields a measure of the former. The parameter a is important because it allows one to evaluate substituent effects on aliphatic reaction rates by a formula analogous to the Hammett equation, or by a bivariate relationship, the Taft-Pavelich equation (Pavelich and Taft, 1957) ... 

Esters are hydrolysed under basic conditions in the presence of quaternary ammonium salts [e.g. 1-7], Microwave activation of basic soliddiquid systems without an added solvent enhances the rate of saponification and the reaction is not affected by steric factors [3], Microwave irradiation has also been used in the hydrolysis and decarboxylation of malonic esters [8] and p-keto esters [9] (>90%). Lactones... 

The polar effects of most m(a and para substituents on the second-order rates of hydrolysis of aromatic esters have demonstrated the marked influence on the energy of activation with little affect to the entropy term. In contrast, ortho substituents alter both the activation energy and the entropy factor. As a general rule, the alkaline hydrolysis of aromatic carboxylic esters is accelerated by substituents with —I, —R effects and is retarded by substituents with +/, +R effects. 

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