Propose a mechanism for the acid-catalyzed reaction of benzald...

Question

Propose a mechanism for the acid-catalyzed reaction of benzaldehyde with methanol to give benzaldehyde dimethyl acetal.

Accepted Answer

Hey everyone and welcome back in this video, we are going to draw a mechanism for the reaction of a seat al to hide with methanol in the presence of an acid. And if we recall 60 reaction is called the formation of an acid ill. So we are going to form and s settle. So let's see how that happens. First of all, we have a seat al to hide. We're going to draw it. That would be our method group bonded to that carbon. We have our carbonell group which contains hydrogen. So that's our seat held hide. Right? And we are reacting it with methanol in the presence of an asset. We call that the welfare step in the acid al formation is the protein ation of the carbonell group. Or specifically this oxygen. So we can use our asset in a generic form hydrogen bonded conjugate base A. And what's happening here is that we have our lone parent oxygen. That can easily be protein ated by the acid. And then we were getting rid of our conjugate base. Now what we get here is our protein ated form of carbon eagle group. So that would be our hydrogen and our carbonell group is now protein ated. So that auction has a formal positive charge. However, this structure can undergo resonance. So we can also indicate brackets to show that its resonance stabilized. And the reason why is because we can easily break that pi bond transfer these electrons in the form of Olympian oxygen and form our carbo Caroline at this position. So let's see what we get. We get our positive charge at this position. And now here we have our alcohol group. Okay, so we got our resonance forms what we want to do. Now we want to look at the second form where we actually have an alcohol group. And this is where methanol comes in. Methanol will essential perform a nuclear attack. Keep in mind that we are trying to form an acid. I'll. So that's why we are looking at this form. So we are taking a molecule of methanol. The lone pair on oxygen essentially attacks the carbo cast iron. So we have our Nikola pilic attack at the electric like center over here and we can drool the result on intermediate. We don't have to worry about this implicit hydrogen. So we may simply skip it. And we can just say that our results on structure would have a metal group on the right, we have our methanol. So that would be O. C. Street still protein ated. So we are going to add that proton, there'll be formal positive charge and oxygen because oxygen has three bonds. And we are going to add our alcohol group because it's the protein ated intermediate. We're going to d protein ate it right. And to do that, we are going to use another molecule of methanol. So if we use another molecule of methanol, the lone pair on oxygen would capture that proton and we would transfer these option hygiene electrons in the form of a lone parent oxygen. And this is how we are getting our first intermediate in this reaction mechanism. So that would be O. H. That metal group on the left and on the right we have our meth oxy group. Okay, so now that we got it, we first of all recall that this is our hemi. Yeah, settle. Right. And because it's not really stable, we are going to continue the reaction in the same pattern so that we exchange our next alcohol group with an appropriate meth oxy group. Right? This is our final goal to get our total. So let's see how that happens. We're going to repeat our process once again and we are going to think about it this way. So the lone pair on oxygen within the alcohol group will once again get protein ated. Because we have that asset H. A. The generic form of our asset that catalyze the reaction after We protein ated, we get our protein ated intermediate. So that would be O. H. T. With a former positive charge on oxygen. We can draw that metal group meth oxy and now we can keep going. Right? So we got it. And we already understand that this is a good leaving group for us because we can easily produce water. So we're getting rid of it. And that was the whole point, right? We protein ated the alcohol to get rid of water and essentially produce our car broke Orion. So now when we have the loss of the leaving group, we are producing our car bo katan, let's draw it, that would be our chain with a positive charge to this position because we lost water. Right? And now what we're going to think we are going to essentially remember that its residents stabilize once again because if you notice there are lone pairs and oxygen which can form a pi bond easily. And of course this carbo Coronas residents stabilized. So we have our double bond and disposition oxygen and then we have a formal positive charge and oxygen. Right? So let's add brackets to indicate residents stabilization here. However, the most important part is the attack of this carbo Karen at this position. So we can essentially get rid of the arrow to make it less confusing for us. The arrow was used for indication of our residents. So let's not worry about it. And now what we're going to do, we are going to think about the nuclear attack by another molecule of methanol, just as we did before the lone pair on oxygen would attack the carb. Okay, Ryan and we would form our second myth oxy group. But before that we still have it protein ated. So we will need to d protein ate it. Right, and when we think about it, we can now draw our structure. So that would be our original structure and now we are bonding, oh which still has hydrogen and a metal group. So now we have a formal positive charge in that auction because it has three bonds. And finally we need to de protein ate it just as we did before. We can use another molecule of methanol CH three oh H. Which will now act as a base. So because it's a proton except that we can simply attack that acidic proton transfer hydrogen oxygen electrons in the form of a lone parent oxygen. And this is how we get our final product. Which is so we can try it. That would be our final product. Right? So we got our mechanism, we can just reveal the steps In the 1st step. We are taking our S et al to hide and we are pro tin ating the oxygen atom from carbon ill using our asset the generic form of an acid H. A. When we protein ate it we understand that it can undergo restaurant. So we are transferring these pi electrons in the form of Olympian oxygen. And we are forming our car broke Orion. We observe the nuclear attack by the methanol molecule. This is where methanol comes in. When we have that attack at the carbo. Karen we have two D. Protein eight, the molecule of methanol to make sure that we get our meth oxide and we D. Protein ated using another molecule of methanol. And this is where we get our hemi acid out because we have our alcohol group and our meth oxy group. So now we are repeating the process. That's it. We're essentially protein ating our alcohol group. Once again using the asset we have the loss of the leaving group which is water. And this is where we get our car broke Orion which is resident stabilized when we get our car broke down. And we have another nuclear attack by the molecule of methanol. And then it's the protein ated using another molecule of methanol which acts as a base to give us our final model. So we can label all of these as our final steps. Let's scroll down the final steps that we discussed are over here we are forming our final acid. I'll when the molecule of methanol attacks the carb. Okay, Ryan it's of course resonance stabilized. We just showed it for complete clarity of the mechanism. And when we have that nuclear filic attack we're just the protein eating that hydrogen to produce our final battle. Now, looking at the mechanism, we can simply conclude that the correct answer choice is a two. This multiple choice question however, if you have any questions, don't have states, leave a comment down below and thank you for watching

Propose a mechanism for the acid-catalyzed reaction of benzaldehyde with methanol to give benzaldehyde dimethyl acetal.

Key Concepts

Acid-Catalyzed Reactions

Nucleophilic Addition

Formation of Acetals

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