Acid-Catalyzed Ester Hydrolysis - Online Tutor, Practice Problems & Exam Prep

Acid catalyzed ester hydrolysis is the reverse of Fischer esterification, converting an ester back to a carboxylic acid using an aqueous acid solution. The mechanism involves protonation of the ester, resonance stabilization, nucleophilic attack by water, and elimination of the alkoxy group. This process highlights the importance of nucleophilic acyl substitution and the role of acids in facilitating hydrolysis, reinforcing the concept that carboxylic acid derivatives can be hydrolyzed to their corresponding acids in the presence of water and acid or base.

General Reaction

Let's talk about a reaction called acid-catalyzed ester hydrolysis. Acid-catalyzed ester hydrolysis is literally just the reverse of Fischer esterification. It's literally just the reverse reaction of producing an ester. You could then hydrolyze that ester back to a carboxylic acid. The general reaction would be that you have your ester, but you react your ester in an aqueous solution with acid and you're going to hydrolyze that ester to a carboxylic acid. Also, guys, if you recall the three rules of NAS, this would be rule number 3, the carboxylic acid conversion that says that pretty much any carboxylic acid derivative in combination with water, acid, or base could turn into a carboxylic acid. Now, what I want to do is go through the mechanism for this. By the way, I already told you it's the exact opposite of Fischer esterification. If you literally wanted to go to the Fischer esterification video and draw out every arrow backwards, every nucleophile backwards, you would get the mechanism. But I know that you guys are going to complain if I don't draw the reverse reaction. Let's go ahead and do that now. I'm going to show you guys exactly how to draw the reverse of Fischer Esterification.

General Mechanism

All right. So let's start off with our ester. And I'm going to go ahead and use H3O+ as my acid. What's going to be the first step? It's going to be protonation. You got it. That's going to give me a compound that looks like this, positive charge. What's my next step? Resonate. So it's resonate. That's going to give me a positive here. My resonance structure. This is a great time to nucleophilically attack. I'm going to get water and my water is going to attack and I'm going to form a molecule. Wait, let me draw my equilibrium arrows. I'm going to form a molecule that looks like this. O H at the top, O R on the side, and water at the bottom. Can you guess what the next step is going to be? What are we trying to get rid of? We're trying to get rid of the OR which means that I want to do a proton transfer specifically to the OR this time. You might be saying, Johnny, how do you know that it doesn't go to the O? Because then that would be the forward mechanism. I'm just trying to go backwards here. I'm trying to get rid of the OR. This OR grabs that H and turns into OR+. Now what happens is I eliminate. I use the electrons from my O to kick out the OR. Oops, that's not going to work. I now have a structure that looks like this. OR+. And now I use water to deprotonate? Water to regenerate my acid. If anything, this serves as extra practice for you for the Fischer mechanism because it is that mechanism, plus my H3O+ and plus my alcohol because I generated one-equivalent of alcohol that I lost. That's really it. Let's move on to the next video.

Here’s what students ask on this topic:

What is acid-catalyzed ester hydrolysis?

Acid-catalyzed ester hydrolysis is a chemical reaction where an ester is converted back into a carboxylic acid and an alcohol using an aqueous acid solution. This reaction is essentially the reverse of Fischer esterification. The mechanism involves several steps: protonation of the ester, resonance stabilization, nucleophilic attack by water, and elimination of the alkoxy group. This process highlights the role of acids in facilitating hydrolysis and the concept that carboxylic acid derivatives can be hydrolyzed to their corresponding acids in the presence of water and acid or base.

What is the mechanism of acid-catalyzed ester hydrolysis?

The mechanism of acid-catalyzed ester hydrolysis involves several steps:

1. Protonation of the ester by the acid, forming a positively charged intermediate.
2. Resonance stabilization of the intermediate.
3. Nucleophilic attack by water, forming a tetrahedral intermediate.
4. Proton transfer to the alkoxy group, making it a good leaving group.
5. Elimination of the alkoxy group, forming a protonated carboxylic acid.
6. Deprotonation of the carboxylic acid to regenerate the acid catalyst and produce the final carboxylic acid and alcohol.

How does acid-catalyzed ester hydrolysis differ from base-catalyzed ester hydrolysis?

Acid-catalyzed ester hydrolysis and base-catalyzed ester hydrolysis differ primarily in their mechanisms and conditions. In acid-catalyzed hydrolysis, an ester is converted to a carboxylic acid and an alcohol using an aqueous acid solution. The mechanism involves protonation, nucleophilic attack by water, and elimination of the alkoxy group. In base-catalyzed hydrolysis (saponification), the ester reacts with a strong base (e.g., NaOH) to form a carboxylate salt and an alcohol. The mechanism involves nucleophilic attack by the hydroxide ion, forming a tetrahedral intermediate, followed by elimination of the alkoxy group and deprotonation of the carboxylic acid to form the carboxylate salt.

What are the key steps in the mechanism of acid-catalyzed ester hydrolysis?

The key steps in the mechanism of acid-catalyzed ester hydrolysis are:

1. Protonation of the ester by the acid, forming a positively charged intermediate.
2. Resonance stabilization of the intermediate.
3. Nucleophilic attack by water, forming a tetrahedral intermediate.
4. Proton transfer to the alkoxy group, making it a good leaving group.
5. Elimination of the alkoxy group, forming a protonated carboxylic acid.
6. Deprotonation of the carboxylic acid to regenerate the acid catalyst and produce the final carboxylic acid and alcohol.

Why is acid-catalyzed ester hydrolysis considered the reverse of Fischer esterification?

Acid-catalyzed ester hydrolysis is considered the reverse of Fischer esterification because it involves converting an ester back into a carboxylic acid and an alcohol, which are the reactants in Fischer esterification. In Fischer esterification, a carboxylic acid reacts with an alcohol in the presence of an acid catalyst to form an ester and water. In acid-catalyzed hydrolysis, the ester reacts with water in the presence of an acid catalyst to form a carboxylic acid and an alcohol. The mechanisms of both reactions are essentially mirror images of each other, with the steps occurring in reverse order.