Ester is reacting with NaOH, and undergoes saponification

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Discussion Overview

The discussion centers around the hydrolysis of esters in the presence of sodium hydroxide (NaOH) and the concept of saponification. Participants explore the mechanisms involved in these reactions, the role of water, and the classification of the reaction as hydrolysis.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions why the reaction is termed hydrolysis when water does not appear as a reactant in the equation involving NaOH.
  • Another participant suggests that the classification as hydrolysis is based on the assumption that the reaction involves ester and water, with the base acting to catalyze the reaction and shift equilibrium by removing one of the products.
  • A different participant clarifies that in saponification, the base is a reactant rather than a catalyst, detailing the mechanism involving the hydroxide ion (OH-) and the formation of a tetrahedral intermediate.
  • This participant also notes that the reaction can occur in the absence of water, but if water is present, it may interact with the alkoxide leaving group, affecting the reaction outcome.
  • Another participant humorously admits to forgetting much of their organic chemistry knowledge, indicating a personal struggle with the topic.

Areas of Agreement / Disagreement

Participants express differing views on the classification of the reaction and the role of water, indicating that multiple competing views remain. The discussion does not reach a consensus on these points.

Contextual Notes

There are unresolved aspects regarding the exact role of water in the reaction mechanism and the implications of classifying the reaction as hydrolysis in the context of saponification.

evry190
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Hi, i was just wondering, when an ester is hydrolyzed with a base, why is it called hydrolysis? Because if an ester is reacting with NaOH, and undergoes saponification then where does the water come in from the reactants side of the equation?
 
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Nice question :smile:

I think it comes from assumption that what is really hapening is the reaction ester + water -> acid + alcohol, base serves two purposes - catalyzes the reaction to speed it up and removes one of the products (acid) to shift the equilibrium. So, if we assume that reaction that is really happening is reaction with water - that's hydrolysis. That makes it easier to classify the reaction, even if technically there is no water in the real reaction equation.

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In saponification, base is not a catalyst... it is a reactant. The reaction mechanism has the OH- from NaOH adding to the ester carbonyl carbon forming the tetrahederal intermediate. The next step is loss of alkoxide (!) followed by the neutralization with whatever acid is closest (either HOH or the nascent acid) but certainly the alkoxide eventually rearranges with something to form the alcohol. You see, it is actually the leaving group alkoxide that deprotonates the product acid. This of course supposes that the reaction occurs in the absence of water. In the presence of water, it is likely that water itself quenches the alkoxide leaving group, reforming an equivalent of OH- which then deprotonates the nascent acid.

Either way, sodium hydroxide reacts on a one-to-one ratio with the ester functionality.

It is called hydrolysis because the net effect of the reaction is the addition of water across the ester bond. That said, in base hydrolysis, one of the hydrogens of the 'added water' is actually the sodium ion unless the reaction's products have been neutralized to reform the acid from the sodium salt.
 
Last edited:


chemisttree said:
In saponification, base is not a catalyst... it is a reactant.

I was trying to not refer to the reaction mechanism, but it seems I have failed

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methods
 


Why were you avoiding the reaction mechanism?
 


Bacause I forgot almost everything I learned studying OrgChem in mid eighties... Two semesters of lab, three semesters of lectures - flushed away, eaten by black hole, as if they never existed :blushing:
 


Oh! Sorry, I thought I was being admonished.:wink:
 

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