Ester Hydrolysis in Basic Solution

Step 1 Nucleophilic addition of hydroxide ion to the carbonyl group 

Step 3 Proton transfers yield an alcohol and a carboxylate ion 

Like the acid-catalyzed mechanism, it has two distinct stages, namely, formation of the tetrahedral intermediate and its subsequent dissociation. Nucleophilic addition to the carbonyl group has a higher activation energy than dissociation of the tetrahedral intermediate step 1 is rate-determining. All the steps are reversible except the last one. The equilibrium constant for proton abstraction from the carboxylic acid by hydroxide in step 3 is so large that it makes the overall reaction irreversible. 

On the basis of the general mechanism for basic ester hydrolysis shown in Mechanism 19.4, write an analogous sequence of steps for the saponification of ethyl benzoate. 

Which ester in each pair would be expected to undergo saponification at the faster rate  

Chapter 19 Carboxylic Acid Derivatives Nucleophilic Acyl Substitution 

Sample Solution (a) p-Nitrophenyl acetate reacts faster. A p-nitrophenyl group withdraws electrons from the ester oxygen which decreases its ability to stabilize the carbonyl group. 

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FIGURE 20 5 The mechanism of ester hydrolysis in basic solution... 

Section 20.11 Ester hydrolysis in basic solution is called saponification and proceeds through the same tetrahedral intermediate (Figure 20.5) as in acid-catalyzed hydrolysis. Unlike acid-catalyzed hydrolysis, saponification is ineversible because the carboxylic acid is deprotonated under the reaction conditions. 

Ester hydrolysis in basic solution is called saponification, after the Latin word sapo, meaning "soap." As we ll see in Section 27.2, soap is in fact made by boiling animal fat with base to hydrolyze the ester linkages. 

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