Carboxylic Acids and their Derivatives 2 Question 5
5. The decreasing order of ease of alkaline hydrolysis for the following esters is
(2019 Main, 10 Jan I)
(a) III $>$ II $>$ IV $>$ I
(b) III $>$ II $>$ I $>$ IV
(c) II $>$ III $>$ I $>$ IV
(d) IV $>$ II $>$ III $>$ I
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Answer:
Correct Answer: 5. (d)
Solution:
- Alkaline hydrolysis of an ester (carboxylic acid derivative) follows acyl $\mathrm{S}_{\mathrm{N}} 2$ mechanism.
Rate of $\mathrm{S}{\mathrm{N}} 2$ mechanism depends on the polarity of $>\mathrm{C}=\mathrm{O}$ group of $-\mathrm{COOR}$ group. Electron withdrawing group $(-R>-I)$ increases the rate of $\mathrm{S}{\mathrm{N}} 2$ reaction whereas electron donating group $(+R>+I)$ decreases the rate of $\mathrm{S}_{\mathrm{N}} 2$ reaction.
Here, the nature of functional groups attached para to the benzene ring are:
$$ -\underset{(-R)}{\mathrm{NO}{2}}>\underset{(-I)}{-\mathrm{Cl}}>-\underset{(+R)}{\mathrm{OCH}{3}} $$
So, the order of hydrolysis will be,
$$ \underset{(-R)}{\mathrm{III}}>\underset{(-I)}{\mathrm{II}}>\mathrm{I}>\underset{(+R)}{\mathrm{IV}} $$
