Propose a mechanism for the following ring-opening transesteri...

Question

Propose a mechanism for the following ring-opening transesterification. Use the mechanism in Problem 21-13 as a model.

Accepted Answer

All right. Hi, everyone. So this question says to propose a mechanism for the acid catalyzed trans reification reaction shown below. So, recall really quickly, right, that an ester can undergo trans estuation reactions in which one alcoy group is substituted for another, no transistor application can occur under both acidic or basic conditions. So, so when the ester of one alcohol is treated with a different alcohol, the two Aloxi groups can interchange but that results in an equilibrium though we can shift the equilibrium towards the extra that we want by using an excess of the desired alcohol. So in this case, we if we were trying to favor the formation of our product on the right side of the screen, we would use an excess of methanol. But in this case, we're going to go ahead and do so in acidic conditions. So in our first step, right, because we have acid in solution, the first step is going to involve the protonation of my carbon. So as a result, right, I'm going to end up with a positive charge on my carbon oxygen because of my extra bond to hydrogen. Now, this intermediate is stabilized via resonance because here, right, the pi bond between carbon and oxygen can break and move towards oxygen to result in a Carbocaine. Right? So here's my ring and now my car boal has become a hydroxy group, but now we have a car in the middle. And here it's also worth mentioning that there is another possibility four resonance, right? Because the Aloxi oxygen can donate a pair of electrons towards the Carbocaine to yield a third resonance structure in which there's a double bond in between oxygen and carbon here inside the ring. So that would give the oxygen inside the ring here, a positive charge. But for the purposes of our reaction, right, the resonance form that is of interest to us is the one involving the carbo car. Because Methanol, our alcohol in question is going to go ahead specifically the hydroxy oxygen here is going to go ahead and attack my carbo ion. So as a result, right, the metox group of methanol is going to go ahead and attach itself to the carbon that was previously a carbo kata. So here's my myth group, but that Metox group is going to have a positive charge on oxygen because my metox oxygen had a hydrogen attached to it when it attacked my Carbocaine. So here right, in order to mitigate the positive charge on our oxygen, the pair of electrons between oxygen and hydrogen is going to go ahead and move towards oxygen. And that proton is going to be disconnected. Now, as a result, we're going to have the following intermediate. Here is my hydroxy group and now here is my neutral myth group. However, right, we're not quite done because by the end of this process, according to the product that we have on the screen, right, we have to have a car bono by the end which at this point, we do not, right. So here at this point, we have to introduce a second equivalent of our asset. So here's my acid and now the oxygen inside of the ring here is what's going to take a proton from my second equivalent of acid. And as a result, I'm going to end up with a positive charge on the oxygen inside the ring because it's going to have an extra hydrogen. So at this point, the bond between oxygen and carbon inside the ring is going to go ahead and break in order to give the pair of electrons between them to the oxygen that now has a positive charge. So as a result, the ring is going to break, I have a neutral hydroxy group at the end. And now the carbon that was formally connected to the hydroxy group inside the ring is a car, right? And this carbo cod ion is once again stabilized via resonant. So I'm going to draw the brackets. And now one possibility is that I can go ahead and form a carbonnel or a double bond between carbon and the oxygen of my hydroxy group. So my first residence structure is going to have a double bond in between oxygen and my hydroxy carbon or hydroxy oxygen, excuse me. And now the positive charge has moved to the hydroxy oxygen. Now, the other possibility is where there could be a double bond in between carbon and my metox oxygen instead. So here I can have a double bond in between my carbon and my metox oxygen. And that is going to give me a positive charge on my metox carbon. But my hydroxy group in this case is going to remain neutral. And now I'm going to close the brackets because at this point, I cannot draw any more resonance structures. But for the purposes of the formation of our product, right, recall that the resonance structure of importance to us is the one in the middle in which we have a proton car bottle. Now that is going to be of importance to us because in this case, we can go ahead and introduce another equivalent of methanol because a second equivalent of methanol can go ahead and act as a base in which the extra proton there is removed. Like so, right, the lone pairs of electrons on my oxygen and methanol can go ahead and take the proton from my protend carbon and yield a neutral carbon at the end. And that is going to result in our final product. And there you have it. So here is our final complete reaction. So now let's go ahead and record our steps. Our first step involved the protonation of the carbonyl in our starting ester with acid resulting in an intermediate that was stabilized via resonance. Now, as a result of resonance, right, a carbo CAD ion formed and then Methanol went ahead and attacked our Carbocaine that resulted in an intermediate in which our metox group was added to our starting carboy carbon. Now to facilitate the opening of the ring itself, we went ahead and pronated in the next step. We went ahead and proton the oxygen inside of the ring with a second equivalent of acid because of that essentially broke the ring and formed another resonant stabilized carbo ion. The resonance resulted in what was almost a car with the exception of an additional proton, but that proton was subsequently removed with a second equivalent or an additional equivalent of methanol to produce our final product at the end. So there you have it. And if you stuck around to the end, thank you so much for watching. And I hope you found this helpful.

Propose a mechanism for the following ring-opening transesterification. Use the mechanism in Problem 21-13 as a model.

Key Concepts

Transesterification

Mechanism of Ring-Opening Reactions

Nucleophilic Attack

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