Pyrrole undergoes electrophilic aromatic substitution more rea...

Question

Pyrrole undergoes electrophilic aromatic substitution more readily than benzene, and mild reagents and conditions are sufficient. These reactions normally occur at the 2-position rather than the 3-position, as shown in the following example.

a. Propose a mechanism for the acetylation of pyrrole just shown. You may begin with pyrrole and the acylium ion, CH3C≡O+. Be careful to draw all the resonance structures of the intermediate.

Accepted Answer

Hey everyone and welcome back in today's video, we're going to solve the following problem. The electra Philip aromatic substitution reaction rate for thiamine is greater than that of benzene. It often reacts under mild reaction conditions, substitution and caffeine occurs preferably at position two. Now troll a mechanism for the settle ation of Taya. Fine. You do not have to show steps for generating the psyllium iron. You may begin with caffeine and a psyllium iron. Okay, so we don't need to worry about the generation of a silly in mind. We can just begin drawing our mechanism. And for these purposes we are going to think about the reaction scheme shown we have our substitution preferably at position two. This is our position number one. Which sulfur and this will be our position number two. We have our substitution. We are going to throw our five numbered ring. Let's indicate the presence of double bonds. Sulfur has two lone pairs the electro file that we're using as a psyllium. And we can simply troll triple bonded oxygen with a formal positive charge is bonded to carbon and that carbon contains a metal group. Recall that in the first step, we have an attack on the electro file. So this pi bond is going to be used for that attack stuff and we are going to attack our electric collect carbon. If we're forming that bond, we need to make sure that we break the adjacent pi bonds were transferring these electrons onto oxygen and this is the structure that we get from here. We can simply enjoy reaction arrow showing that this is our attack on the electro file. Let's draw our five member ring. And now we understand that position. Number two will be occupied by our carbon steel group. So that will be our carbon. Right? We have added it now oxygen will be double bonded and there is that metal group bonded to that carbon. Now the positive charge is over here because we're breaking the pi bonds. So there will be a lack of electrons in this position, Sulfur has two lone pairs. This is our in immediate right? We got our sigma complex. This is the first structure we need to think about the residents whenever we are drawing mechanisms. And first of all, we notice that there's this adjacent pipe on that can be used to de localize the pi electrons and get a positive charge at a different position over here. So let's indicate the residence arrows as well because this is where our residents starts. So we're going to add brackets. Right? And now we can add our first arrow. The first era will illustrate the formation of the 2nd residence form. So we are going to draw the five member ring once again. However, now our pi bond is over here and then we have a lack of electrons at the carbon atom adjacent to sulfur. So we that's why we have a positive charge and let's add our substitute print. So this is our second residence form. Let's label it. We got our second residence form and we need to think of any other possible residence and sulfur helps us achieve that, right? That's why substitution takes place preferably at position number two. Because now the sigma complex is actually stabilized by the D localization of the lone pair of sulfur. So we are going to form our pipe on in this position. This gives us an additional resonance form which stabilizes the sigma complex. Let's draw our 3rd residence form. If we begin by drawing the five member dring, we noticed that the newly formed pi bonds between the carbon and sulfur. So now because sulfur has three bonds, it would correspond to a formal positive charge. We're going to add our substitute wint And that will be our 3rd residence form. Because we are done with residents. We can essentially at a closing bracket, there is no more way to de localize that positive charge, essentially. There's no more adjacent pi bond that would allow us to de localize the positive charge. So we are done with our residence forms. So we just want to make sure that we get our product from here. Let's recall that whenever we are adding our electro file, we want to remove that hydrogen. It's implicit let's think about the third resonance form. For simplicity, it's perfectly fine. If you choose another form, you just want to make sure that you're showing the deep rotation stuff correctly. And if we expand that hydrogen, it will be essentially captured by a base. And we are going to we're going to transfer these carbon hydrogen electrons into a newly formed pi bon over here. But if we do so, we have to break the adjacent pi bonds, we're going to transfer these pi electrons over here. And similarly we have to break another adjacent pi bond. And the only way to do that is to restore these two single bonds on sulfur and place the pi electrons in the form of a lone pair on sulfur. So from here we can essentially show that we are getting our final product because the arrows illustrate the formation of the final structure and we control a single arrow And this is our final product that will be five member grain. Now let's add our substitution. And from here we notice how we are forming this five on over here, another pi bond between these two carbon atoms and our sulfur gets its lone pair back so it now has two lone pairs. Once again, this is indeed our final product, meaning the mechanism that we have proposed is sensible. We can label our mechanism as our final answer because we are asked to draw the mechanism and based on what we see on this mechanism, we can essentially conclude that the correct answer to the multiple choice question is C However, if you have any questions, don't hesitate to leave a comment down below and thank you for watching

Key Concepts

Electrophilic Aromatic Substitution (EAS)

Resonance Structures

Acylium Ion

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