I had a doubt while reading about the saponification reaction of esters in basic medium. What is the molecularity of the reaction and why?

P.S - I think its bimolecular and heres why -

The RDS of the reaction is the formation of the tetrahedral intermediate after the attack of the OH ion. In this step both the ester and the hydroxide ion are involved (The ester is shifting the carbonyl bond electrons to the oxygen and the hydroxide is attacking the electrophilic carbon). Therefore, the molecularity should be two.


You should know that the molecularity is not relevant in your case and has no significance for multi-step reactions, it is just applicable to elementary reactions. The relevant parameter for multi-step reactions is the, reaction order wich is originally an experimental parameter indirectly related to the molecularity of all elementary reactions that makeup the reaction mechanism.

So for saponification the proposed mechanism includes 3 steps, the last one of them is irreversible and controls the reaction. The order of the reaction is considered 2, the molecularity for the step that you have mentioned is also 2 but it is just one step in the mechanism.

Also, there are two types of mechanisms through which this reaction takes place: a base catalyzed mechanism (with simple base or alkoxides) and an acid catalyzed mechanism.


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