Base-Catalyzed Ester Hydrolysis (Saponification)

Concept: Concept: Mechanism

Video Transcript

In this video, I’m going to teach you about the base-catalyzed ester hydrolysis reaction or better known as the saponification reaction. Saponification is just a base-catalyzed hydrolysis of an ester, meaning that if I have an ester and I expose it to water and base as a catalyst, I’m going to hydrolyze that ester. We would expect to get a carboxylic acid. But if all you have is base present, you're actually going to yield a carboxylate anion. That makes sense because there would be nothing around to protonate it. In the absence of a protonation step, you actually should end up with an O negative carboxylate, or what’s also called a carboxylate salt because it has that full negative charge.
Just so you know, that is something that you should be aware of that you need the protonation step to get to the carboxylic acid. Now we’re just going to go through the mechanism. I'm not sure if you’ve heard me say this before but in general in organic chemistry, a base-catalyzed mechanism is always easier than an acid-catalyzed mechanism. This is actually going to be a very straightforward mechanism. Let's go ahead and assume that I'm going to be using water and base, so I’m just going to use OH negative. That’s my base. In my first step, I’m going to do a nucleophilic attack because this is already strong enough to attack that partial positive. I don't need to do a protonation and draw a resonance structure to make that carbon active. It's already acted because I’ve got a negatively charged nucleophile. I’m going to get a tetrahedral intermediate. We've seen this before. I’ve got O negative. I’ve got OR. I’ve got OH, and I’ve got my carbon.
Then we're going to go ahead and we're going to reform the double bond. We’re not going to protonate because this is nucleophilic acyl substitution. This is an NAS reaction. We're going to kick out a leaving group. Which one do we kick out? Do we kick out the OH or OR or the R-group? It really depends on the way you want to draw it and which direction you’re trying to go in. Since I’m trying to go towards hydrolysis, I kick out the OR. What that's going to give me is a molecule that looks like this. Now it’s a carboxylic acid. You might be thinking, “Oh I did get a carboxylic acid.” But no, because you're going to have OR negative around now and that OR negative will deprotonate it. Then we're just going to get our carboxylate as a product plus alcohol. That's where it ends. That’s the end of the mechanism.
If you protonate it afterwards, then you can get a carboxylic acid but you need that protonation step to get there. As you guys can see, super easy mechanism compared to the acid-catalyzed version. Let's move on.