Acid-catalyzed transesterification:
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Question

Acid-catalyzed transesterification:

Complete the mechanism for this acid-catalyzed transesterification by drawing out all the individual steps. Draw the important resonance contributors for each resonancestabilized intermediate.

Accepted Answer

All right. Hello everyone. So this question is asking us to provide the detailed mechanism for the following acid catalyzed trans ester application. If the step involves a resonance stabilized intermediate show the important resonance contributor. So in this particular case, recall that trans ester application revolves around a reaction using one ester with a given oxy group being converted into a different ester with a different oxy group. And that's done by reacting our initial ester with an alcohol because it's the alcohol that provides the new Coxie group, the old oxy group. On the other hand, is detached eventually from the car bono and becomes its own alcohol by-product. So with that in mind, here, we have isopropyl propionate as our starting material reacting with ethanol to form ethyl propionate and propane tool. So up on the screen, I went ahead and drew our isopropyl ester just to go ahead and begin the mechanism. So in the first step of the mechanism, we have the interaction between our starting ester and our acid catalyst, which in this case is HCL. So the car bottle oxygen will be proton buy HCL. So here's a long pair of electrons from that carbon oxygen taking a proton from HC which generates chloride ion as an intermediate or rather as a by-product. So as a result, we end up with this pronated carbel intermediate and this intermediate is in fact, resonance stabilized. So let's just quickly review the important resonance forms of this species, right. So first, the pi electrons of the car bottle itself can move towards oxygen to create an extra lone pair. Now this ultimately makes that oxygen neutral. It makes it a hydroxy group specifically, but we're left with a carbo cion as a result of this. And so the neighboring oxygen from the alcoy group can donate a lone pair of electrons between itself and the carbo Kia. And so this ultimately makes our carbon here neutral, but the oxygen itself of the oxy group becomes positive due to that double bond with carbon. However, for the next step of this mechanism, the most relevant resonance form is going to be the one in which there is a cruel cat out. So just to go ahead and make the mechanism a little bit less confusing, I'm going to go ahead and use red arrows to showcase resonant movements. So once again, I'm going to use these red arrows just to distinguish between electron movements that are because of residents. And that's because we can now proceed with the next step of our mechanism. All right. So as mentioned before, we can now proceed to the next step of the mechanism I just wanted to very quickly move some of the drawings that I did previously for the sake of space. So now using the resonance form that shows our carbo cion ethanol is going to behave as a nucleo and it's going to attack that position directly. So the oxygen atom from ethanol is going to now establish a bond between itself and that carbo cat isle. So here's our hydroxy group and here's that proton alcohol. So now it's worth mentioning that because ethanol was neutral at the time that it attacked carbon oxygen from ethanol is now going to have a positive charge. And so the next step is just meant to alleviate that positive charge right, to remove it. So to do that, we can have another equivalent of ethanol that is available in solution going ahead and detonating that oxygen. So now we have a neutral ay group attached instead. And so at this point, our goal is to go ahead and remove that initial epoxy group from the starting ester in this case, the iso oxy group. So here, once again, we're going to need to use the acid that's in solution right now. So we can take another equivalent of HCL and have that isopro boxy oxygen get proton it rather. So here's that isopro boxy oxygen now with a positive formal charge and because it has a positive formal charge, it's now going to be an excellent Carbo Kia or excuse me, an excellent lead group I should say. And that's because there is an incentive for this isopro boxy oxygen to detach itself from carbon specifically when it does. So it takes a loan pair of electrons with it, thereby escaping as our alcohol by-product as isopropanol specifically. And so at this point, we're left once more with a cruel cion, but now notice how we have that epoxy group attached to carbon as opposed to the isopro boxy group that we had in the beginning. So here isopropanol is our by-product and now that we have a crumble c, this cruel cion does have a few resonance forms as discussed in the prior steps. So let's go ahead and talk about those resonance forms. Now for the step of the mechanism for the first resonance form, I'm going to go ahead and showcase it using red arrows. So first, we can have the epoxy oxygen, for example, establish a double bond between itself and the carbo car. So this is ultimately going to make carbon neutral, but oxygen is going to be positive instead. So in addition to this resonance form, there is one more available which I'm going to illustrate using blue arrows. So in this additional resonance form, the hydroxy oxygen can instead donate a long pair of electrons to establish a double bond between itself and carbon. And so this is going to result in a proton car bal that is going to be relevant for us in this last step of the mechanism. Now. So here are the three resonance forms, but that last one, once again, with that pronated car bono is going to be the one that's relevant for this step. And that's because after that proton, carbot is dep proton, then our final product is going to be generated. So let me scroll down over here. So just to reiterate now for the final step, we can have another equivalent of ethanol available to detonate and there you have it. So with that, our mechanism is now complete. One more quick note that I did not mention earlier on in this video, but this process is reversible, hence the equilibrium arrows used throughout. And so with that being said, if you watch this video all the way through, thank you so very much. And I hope you found this helpful.

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<IMAGE> Acid-catalyzed transesterification:

Complete the mechanism for this acid-catalyzed transesterification by drawing out all the individual steps. Draw the important resonance contributors for each resonancestabilized intermediate.