On this page, we're going to discuss a reaction called transesterification. Base catalyzed transesterification occurs when an ester is exposed to an alkoxide base with a dissimilar alkyl group. If I'm using an alkoxide base, let's say that I was in a basic environment because it's base catalyzed. Imagine that my alcohol instead of having an R group, the R group that I have on my alkyl group, I have R prime. R prime just means it's different. It's something different. Maybe one's a methyl and one's an ethyl. What's going to wind up happening is after it reacts, we're going to wind up getting another ester. It seems like nothing changed. But wait, the R group is going to be different. I'm going to get one R group transferring with another or substituting with another. This happens because in equilibrium, all of these OH groups are going to be constantly substituting back and forth. If you have different alkyl groups in different positions, they're going to wind up blending together and you're going to wind up getting OH groups of both types on your ester. That's very problematic because when you have an ester, you want to make sure that it has the same alkyl group. You don't want a bunch of different random alkyl groups. There is one way this can be avoided. The way this can be avoided is simply to only expose esters to alkoxides with the same R group. I'm just going to bring this reaction down a little bit. Imagine that now I have, let's say R₁ and I'm exposing it to an alkoxide that is O-R₁ negative. What's going to happen? In equilibrium, it's going to switch, and it's going to perform this whole mechanism that I'm going to show you. But are we going to be able to actually tell that it's happening? The answer is no reaction. Does it have any effect on my product? No, because substitutions are taking place. But since my R groups are identical, I won't be able to notice this difference. I won't be able to notice that a reaction is actually taking place because it's not really mattering for my reaction. The only time it would matter is if I have a different R group. Let's say it was R₁ and R₂. Now, I'm going to get a mixture of R groups transesterifying. In this next video, I'm going to show you guys the mechanism. Heads up, it's easy.
Transesterification - Online Tutor, Practice Problems & Exam Prep
Base-catalyzed Transesterification occurs when an ester is exposed to an alkoxide base with a dissimilar alkyl group. Now, do we think this a good thing? Stay tuned for the answer!
General Reaction
Video transcript
General Mechanism
Video transcript
All right. Let's say that I do mess up and I have my methyl ester and I forget that it's a methyl ester and I expose it to ethoxide, so CH3CH2O-. I wasn't supposed to do that. I should have only used methoxide here because I know methoxide would avoid this situation. But let's say I use ethoxide by mistake. This is what happens. You wind up getting a nucleophilic acyl substitution. You wind up forming a tetrahedral intermediate. And that's going to have now OCH3, OCH2CH3. What happens? Which group gets kicked out? In my SN2 reaction, it could be either one. The point here is that it's going to be a mixture of both, so I'm going to wind up getting 2 different esters. I'm going to wind up getting the ester that forms when it kicks out the methyl. It's going to wind up being an ethyl ester. But I'm also going to get the one that forms if it kicks out the original molecule. Then that would be a methyl ester. Anyway, you should never draw multiple arrows like that on the same reagent. That's kind of me just showing you conceptually what's going on. Let's just say that you are going forward. You're transesterifying. You're kicking out the OCH3. Well then you're going to get this as a product. You're going to wind up getting an ethyl ester as a product. Again, this is problematic and this is going to be important later on. There's going to be more reactions that we're going to use esters for. It's going to be important to always use an alkoxide of the same base, of the same R group. If I can keep those R groups consistent, then transesterification doesn't matter to me anymore because even if the reaction is taking place, I can't appreciate it because nothing is actually happening. I hope that made sense guys. Really easy mechanism. Let's move on to the next video.
Mechanism:
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