Ester is reacting with NaOH, and undergoes saponification

[evry190]

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?

 

[Borek]

Nice question

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.

--

 

[chemisttree]

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.

 

[Borek]

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

--
methods

 

[chemisttree]

Why were you avoiding the reaction mechanism?

 

[Borek]

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

 

[chemisttree]

Oh! Sorry, I thought I was being admonished.