Question

Which of the following sequences of rate of alkaline hydrolysis of esters is correct:
A.${\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{COOC}}{{\text{H}}_{\text{3}}}{\text{ < }}{\left( {{\text{C}}{{\text{H}}_{\text{3}}}} \right)_{\text{2}}}{\text{CHCOOC}}{{\text{H}}_{\text{3}}}{\text{ < }}{\left( {{\text{C}}{{\text{H}}_{\text{3}}}} \right)_{\text{3}}}{\text{CCOOC}}{{\text{H}}_{\text{3}}}$
B.${\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{COOC}}{{\text{H}}_{\text{3}}} > {\left( {{\text{C}}{{\text{H}}_{\text{3}}}} \right)_{\text{2}}}{\text{CHCOOC}}{{\text{H}}_{\text{3}}} > {\left( {{\text{C}}{{\text{H}}_{\text{3}}}} \right)_{\text{3}}}{\text{CCOOC}}{{\text{H}}_{\text{3}}}$
C.${\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{COOC}}{{\text{H}}_{\text{3}}} > {\left( {{\text{C}}{{\text{H}}_{\text{3}}}} \right)_{\text{2}}}{\text{CHCOOC}}{{\text{H}}_{\text{3}}} < {\left( {{\text{C}}{{\text{H}}_{\text{3}}}} \right)_{\text{3}}}{\text{CCOOC}}{{\text{H}}_{\text{3}}}$
D.${\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{COOC}}{{\text{H}}_{\text{3}}} < {\left( {{\text{C}}{{\text{H}}_{\text{3}}}} \right)_{\text{2}}}{\text{CHCOOC}}{{\text{H}}_{\text{3}}} > {\left( {{\text{C}}{{\text{H}}_{\text{3}}}} \right)_{\text{3}}}{\text{CCOOC}}{{\text{H}}_{\text{3}}}$

Answer 1

Hint:When an ester comes into contact with water, it breaks down into a carboxylic acid and an alcohol. To answer this question, you must recall the alkaline hydrolysis reaction of an ester. Lesser the substitution, more the rate of reaction.

Complete answer:
Since the hydrolysis of esters is generally very slow as compared to those of other carboxylic acid derivatives, the reaction is generally catalyzed by an acid or a base.
When a hydroxide nucleophile adds to the carbonyl carbon of the ester, a tetrahedral intermediate is formed where the $s{p^2}$ carbon in the reactants becomes $s{p^3}$ in the intermediate. This attack is nucleophilic addition to the carbonyl carbon. The carbon atom is highly electrophilic due to the presence of two oxygen atoms attached to it. So the rate of this step depends on the electrophilic character of the carbon atom.
In the next step, the unstable tetrahedral intermediate undergoes reformation and the alkoxy group leaves the ester forming an acid and alcohol as product. The rate of this reaction depends upon the strength of the leaving group.
The rate of the overall hydrolysis reaction depends on the electrophilicity of the carbon atom and the nature of the leaving group. In the given esters, the leaving groups are the same. The electrophilic nature varies according to the inductive effect of the alkyl groups. Tertiary alkyl group is most electron donating and thus that carbon is least electrophilic.
Thus, the order is ${\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{COOC}}{{\text{H}}_{\text{3}}} > {\left( {{\text{C}}{{\text{H}}_{\text{3}}}} \right)_{\text{2}}}{\text{CHCOOC}}{{\text{H}}_{\text{3}}} > {\left( {{\text{C}}{{\text{H}}_{\text{3}}}} \right)_{\text{3}}}{\text{CCOOC}}{{\text{H}}_{\text{3}}}$

Hence, the correct order is B.
Note:
Esters are one of the most important functional groups and are widely distributed in various natural substances like fruits, flowers, etc. Fats and oils too are esters of higher fatty acids of glycerol and are the main constituents of plant and animal tissues for their fundamental energy and functions.