Base-catalyzed hydrolysis of esters

Fig. 7.2. a) The most common mechanism of base-catalyzed ester hydrolysis, namely specific base catalysis (HCT catalysis) with tetrahedral intermediate and acyl cleavage. Not shown here are an W mechanism with alkyl cleavage observed with some tertiary alkyl esters, and an 5n2 mechanism with alkyl cleavage sometimes observed with primary alkyl esters, particularly methyl esters, b) Schematic mechanism of general base catalysis in ester hydrolysis. Intermolecular catalysis (bl) and intramolecular catalysis (b2). c) The base-catalyzed hydrolysis of esters is but a particular case of nucleophilic attack. Intermolecular (cl) and intramolecular (c2). d) Spontaneous (uncatalyzed) hydrolysis. This becomes possible when the R moiety is... 

Before we turn to discussing neutral and base-catalyzed hydrolysis of ester functions, we need to reflect on what structural features determine how good a leaving... 

Now that the steric parameter can be evaluated, the inductive parameter is available. Taft noted that the transition-state structures for acid- and base-catalyzed hydrolysis of esters (15 and 16, respectively) differ from each other by only tiny protons. Therefore the steric effect of a substituent should be approx-... 

Esters react by both acid and nucleophile initiated mechanisms. Hydrolysis of esters by acid catalysis is exactly the reverse of the mechanism for the acid-catalyzed esterification of a carboxylic acid. Base-catalyzed hydrolysis of esters is called saponification. Hydroxide attacks to form a tetrahedral intermediate. Loss of alkoxide ion then occurs. The alkoxide neutralizes the resulting carboxylic acid to form the salt. 

It is not very surprising that the evaluation of the heats of activation of various enzyme-catalyzed ester hydrolysis reactions has proved to be uninformative, the values being very close to those of base-catalyzed hydrolysis of esters. It may, however, prove very useful when the work on the differentiation between separate steps, described in this pajjer, can be extended by a study of the effect of temperature on the different steps. 

It has been observed that the rate of the base-catalyzed hydrolysis of esters of phtha-lic acid decrease with the size of the alcohol moiety, Table 8.2. A Taft-Hancock relation relates rate constant, k, to the inductive effect of these substituents cr, the steric effect and the rate constant feo of the dimethyl ester. The coefficient 4.59 corresponds to the p of the Hammett equation and reflects the sensitivity to... 

TABLE 8.2 Base-Catalyzed Hydrolysis of Esters of Phthalic Acid at 30°C... 

Volumes of Activation for the Acid-Catalyzed and Base-Catalyzed Hydrolysis of Esters and Amides... 

The base-catalyzed hydrolysis of esters is accelerated by the presence of 10 M POE(9)nonylphenylether nonionic surfactant, unaffected by... 

There are two kinetically indistinguishable mechanisms for the general-base-catalyzed hydrolysis of esters, both of which would show acyl carbon to oxygen bond cleavage. What are these ... 

Base-Catalyzed Hydrolysis of Esters Saponification (section 21.11)... 

As an alkaline environment is encountered during interesteriflcation, the catalyst (which is nucleophilic) attacks the slightly positive carbonyl carbon at one of three fatty acid-glycerol ester bonds to form a tetrahedral intermediate. A fatty acid methyl ester is released, leaving behind a glycerylate anion. Kinetics of base-catalyzed hydrolysis of esters are second-order reactions dependent on both ester... 

The rates of hydrolysis of esters and related compounds are catalyzed by specific acid and specific base catalysts. Fine details of general mechanisms of specific acid and specific base catalyzed hydrolysis of esters are shown in Scheme... 

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