Chapter 20, Problem 20.33P

Interpretation Introduction

Interpretation:

Which of the two esters shown will undergo saponification faster is to be determined.

Concept introduction:

Saponification is essentially the hydrolysis of an ester under strongly basic conditions. The actual mechanism consists of an initial nuleophilic addition of hydroxide ion to the carbonyl carbon of the ester. This is followed by elimination of the $-\text{OR}$ group of the ester to form the carboxylic acid. Under the strongly basic conditions, this is irreversibly deprotonated to form the metal carboxylate salt. This reaction is known as saponification

If the carboxylic acid is to be recovered, an acid workup must follow.

The first step in the saponification reaction, nucleophilic addition, is the rate detemining step of the mechanism. The overall reaction rate, therefore, depends on the stability of the tetrahedral intermidiate, which in turn depends on how easily the hydroxide ion can add to the csarbonyl carbon. The higher the partial positive charge on the carbonyl carbon, the easier the nucleophilic addition of the hydroxide ion. The partial positive charge of the caronyl carbon depends primarily on the electron-withdrawing/donating property of the R group of the acid (alkanoate) part of the ester. If this group is electron-withdrawing, it will increase the partial positive charge on the carbonyl carbon and make it easier for the hydroxide ion to attack it.