what is the role of sodium hydroxide in saponification?

Many famous textbooks in Organic Chemistry and Webpages of Universities state that NaOH acts as one of the reactants (i.e. the reagent) in saponification (base hydrolysis of ester). The mechanism of nucleophilic acyl substitution, in which the OH- ion from NaOH acts as the nucleophile.

The mechanism of base hydrolysis of an ester (saponification) is outlined below. The mechanism is supported by experiments using 18O labelled compounds and esters of chiral alcohols.


圖片參考：http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch20/sapmech.gif


Obviously, OH- ion from NaOH takes place in the reaction, and it acts as one of the reactants.

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Some textbooks in Hong Kong and some webpages state that NaOH acts as a catalyst. It is incorrect. None of the textbooks and webpages can provide acceptable mechanism to support their point of view.

Why NaOH does not act as a catalyst in saponification ?

Firstly, catalyst is defined as “a substance which can alter the rate of a reaction, but is chemically unchanged at the end of the reaction.”
According to the equation of saponification :
RCOOR’ + NaOH = RCOONa + ROH
NaOH does chemically change at the end of the reaction, it is not a catalyst.

Secondly, some webpages state that the saponification is divided into the two reactions. The first reaction is the hydrolysis of the ester to carboxylic acid and alcohol, and the reaction is catalyzed by H+ ion.
RCOOR’ + H2O = RCOOH + ROH (acid catalyzed)
The RCOOH formed is neutralized by NaOH to give RCOONa. The equilibrium position of the above reaction is thus shift to the right to make the reaction complete.
However, the above explanation can only explain that NaOH acts as a base, but not a catalyst. In fact, the above explanation is incorrect, because the concentration of H+ ion in NaOH solution is too low to act as a catalyst.



2007-04-08 00:55:41 補充：
Saponification means base hydrolysis of esters. The molecules of fats and oils are triglycerides (containing 3 ester groups). Therefore, saponication means base hydrolysis of esters, including base hydrolysis of fats and oils.

Although I don't know the exact answer, if you ask me to make an estimation, I will guess it would be an essential reactant.

During saponification, sodium hydroxide acts as a substance that provides an alkaline condition to the fat and thus the fat is hydrolysed to give a soap. This process is very important in making soaps since the soap made out contains an anionic head which is soluble in water can therefore has detergent properties.

By the way, I think that sodium hydroxide would not act as a catalyst. During saponification, the process will take place if there are the presence of sodium hydroxide , it's no need to give a catalyst.