Are you having deja vu? You should be! We are learning a mechanism that is the exact opposite of Fischer Esterification.That means instead of going from a Carboxylic Acid to an Ester, we are hydrolyzing an Ester to a Carboxylic Acid.
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. You’re going to hydrolyze that ester to a carboxylic acid. Also if you recall, the three rules of NAS. This would be rule number three. The carboxylic acid conversion that says that pretty much carboxylic acid derivative in combination with water or 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 Fisher 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 Fisher esterification.
Alright, so let's start off with our ester, okay? And, I'm going to go ahead and use H3O plus as my acid, okay? So, what can be the first step? okay it 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, draw my resonance structure and this is a great time to nucleophilically attack. So, I'm going to get water and my water is going to attack and I'm going to form some molecule, wait, let me draw my equilibrium arrows, I'm going to form a molecule, it looks like this OH at the top, OR on the side and water at the bottom, can you guess what the next step is going to be? So guys, 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, okay? if you say, Well, Johnny, how do you know that it doesn't go to the OH? Well, because then that would be the forward mechanism, okay? So, I'm just trying to go backwards here. So, I'm trying to get rid of the OR. So, this OR grabs that H and turns into OH R positive, okay? Now, what happens is I eliminate, I use the electrons from my O to kick out the OR we, that's not going to work, and I now have a structure that looks like this OH positive ROH and now I use what to deprotonate? water, to regenerate my acid, okay guys? So, if anything this serves as extra practice for you for the Fischer esterification mechanism because it is that mechanism, okay? Plus my H3O plus and plus my alcohol, right? Because I generated one equivalent of alcohol that I lost, okay? So guys, that's really it, let's move on to the next video.