Aspirin + Sodium Hydroxide Titration

Question

I am trying to find how this reaction has a molar ratio of 1:2 (aspirin: NaOH) as my teacher said but can't find the equation!

Aspirin (C9H8O4)



Answer 1

Aspirin is the generic name for the compound called acetylsalicylic acid. It is a monoprotic acid and thus can be rewritten as HC₉H₇O₄ (with the H⁺ being in front the acetylsalicylate anion (C₉H₇O₄^- )).

If acetylsalicylic acid is mixed with sodium hydroxide, we get the following balanced reaction:

HC₉H₇O₄ + NaOH ---> H₂O + Na(C₉H₇O₄)

It looks like there is a 1:1 ratio between aspirin and sodium hydroxide and if this were a simple titration, I'd say that would be true--the OH from the sodium hydroxide would immediately react with the H from the acid. But, I think functional groups here play a significant role and thus by extent the bonding mechanisms of functional groups.

Aspirin has a carboxylic acid functional group as well as an ester functional group. When we are dealing with esters, it is important to remember that in the presence of water, esters undergo hydrolysis and form a carboxylic acid functional group.

So, for the reaction given above, the OH will react immediately with the carboxylic acid functional group of aspirin and yield a water molecule. However, this water molecule will then hydrolyze the ester functional group of aspirin and thus yield another carboxylic acid group on aspirin. As such, we end up having two carboxylic acids groups that we have to neutralize in aspirin--one from a first stage and another from a second stage. We therefore need 2 NaOH atoms in order to effectively neutralize aspirin.

Understanding this, I believe that is your teacher's rationale for why the molar ratio between aspirin:NaOH is 1:2 despite the balanced reaction showcasing a 1:1 ratio.

I hope this is insightful!