Animated Solution for Chemistry - Organic Chemistry: The decreasing order of ease of alkaline hydrolysis for the following esters is

A Tale of Substituent Effects

Alkaline hydrolysis of esters, commonly known as saponification, is a fundamental reaction in organic chemistry. It proceeds via the acyl $S_{N}2$ mechanism, where a hydroxide ion ($ext{OH}^{-}$) acts as a nucleophile and attacks the electrophilic carbonyl carbon of the ester.

The rate of this reaction is entirely dependent on the initial nucleophilic attack. For the hydroxide ion to attack efficiently, the carbonyl carbon must be highly electron-deficient. In other words, the greater the partial positive charge ($\delta +$) on the carbonyl carbon, the faster the reaction will proceed.

EWG vs EDG

When we introduce substituents onto the benzene ring of ethyl benzoate, they dramatically alter the electron density at the carbonyl carbon through inductive ($I$) and resonance ($R$) effects.

Electron-Withdrawing Groups (EWG), such as the nitro group ($-{\text{NO}}_2$), pull electron density away from the ring and the ester group. This intensifies the $\delta +$ charge on the carbonyl carbon, making it a highly attractive target for the incoming nucleophile. Consequently, EWGs significantly increase the rate of hydrolysis.

Conversely, Electron-Donating Groups (EDG), like the methoxy group ($-{\text{OCH}}_3$), pump electron density into the ring via resonance. This influx of electrons neutralizes the $\delta +$ charge on the carbonyl carbon, making it less electrophilic and thereby slowing down the nucleophilic attack.

Let's evaluate the specific substituents provided in the problem, all located at the para position:

1. Molecule III ($-N{O}_2$): The nitro group is a powerful EWG, exerting both strong $-R$ and $-I$ effects. It creates the most electron-deficient carbonyl carbon, making this ester the most reactive. 2. Molecule II ($-Cl$): Halogens are unique; they possess a $+R$ effect due to their lone pairs but a stronger $-I$ effect due to their electronegativity. The net result is electron withdrawal, making it more reactive than the unsubstituted ester, but less reactive than the nitro derivative. 3. Molecule I ($-H$): The unsubstituted ethyl benzoate serves as our baseline for comparison. 4. Molecule IV ($-OC{H}_3$): The methoxy group is a strong EDG due to its dominant $+R$ effect, which easily overpowers its weak $-I$ effect. It enriches the carbonyl carbon with electron density, making this ester the least reactive of the group.

By comparing the electronic effects, we establish the order of reactivity as: $-{\text{NO}}_2>-\text{Cl}>-\text{H}>-{\text{OCH}}_3$. Therefore, the decreasing order of ease of alkaline hydrolysis is III > II > I > IV.