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Video transcript
In the last video we looked at our Friedel-Crafts alkylation. In this video, we're going to an acylation, which is very similar to the alkylation. We start off with benzene, and to benzene we add an acyl chloride. And so this right here, you could think about as an acyl group. We're also going to use aluminum chloride once again as our catalyst, and you can see the acyl group has substituted in for one of the aromatic protons, and so that's our electrophilic aromatic substitution reaction. The mechanism for an acylation is similar to an alkylation, although there is an important difference, but they start off the same in that aluminum chloride is going to function as a Lewis acid and accept a pair of electrons. And so this lone pair of electrons on this chlorine, you could think about that chlorine donating that pair of electrons and the aluminum accepting that electron pair. And so if I go ahead and draw the result of that Lewis acid base reaction, we have our carbonyl, we have our chlorine attached to our carbonyl carbon. The chlorine has two lone pairs of electrons. It's now formed a bond with the aluminum, and the aluminum is bonded to three other chlorines. And I'm not going to draw in the lone pairs of electrons on those other chlorines just to save time, but the aluminum gets a negative 1 formal charge, and this chlorine now has a positive 1 formal charge. So to highlight our electrons, these electrons right here in magenta, are forming a bond between the chlorine and the aluminum like that. So in order to find our electrophile, you could think about these electrons in here kicking off onto the chlorine. And so I'm taking a bond away from that carbonyl carbon, and so if I take a bond away from that carbonyl carbon, that carbon is now positively charged. That carbon is still double-bonded to an oxygen, and that carbon is bonded to an R group. And so we've created an acyl cation. And we can think about this acyl cation as being the electrophile in our mechanism for electrophilic aromatic substitution. This cation is resonance stabilized. I could take a lone pair of electrons here on this oxygen and move them into here so I could draw a resonance structure where now the carbon would be triple-bonded to this top oxygen here. This top oxygen would still have a lone pair of electrons and have a plus 1 formal charge like that. And this carbon is still bonded to an R group, so I am saying that this lone pair of electrons here on this oxygen can move into here like that. And this resonance stabilization of the acyl cation is one difference between an acylation and an alkylation. Because our cation in an acylation is resonance stabilized there is no rearrangement, and that's different from what we saw on our alkylation reaction. We formed a carbocation that was capable of rearranging to form a more stable carbocation in the previous video on alkylation, and so that made it a little bit difficult to control the types of products that we got. And so with the acylation there is no rearrangement, and again, it's due to this resonance stabilization of our acyl cation here. So we would also form this complex over here where the aluminum is bonded to four chlorines. So we could think about this chlorine now as having three lone pairs of electrons around it, so I'm going to highlight those electrons in red. So these electrons in here kick off onto the chlorine like that, and once again, we still have a negative 1 formal charge on this aluminum like that. So the catalyst has generated our electrophile, and now we can show our electrophile, our acyl cation reacting with a benzene ring. And for that mechanism, you could show either one of these. You could show either one of these resonance forms reacting with your benzene ring. I'm just going to take the one on the right and the one with the positive charge on the carbon. So we have our benzene ring, and we have one of the protons on our benzene ring like that. And I'm choosing the resonance structure on the right so I'm going to have a plus 1 formal charge on my carbon. And I'm going to have an R group attached to that carbon as well. So we have a nucleophile electrophile situation. So once again, negative charges attract to positive charges. These pi electrons are going to function as a nucleophile, attack our electrophile, and so we can add our electrophile onto our benzene ring. And so once again, I'm going to show our electrophile adding to the top carbon here. So the top carbon has a hydrogen, and now it's going to form a bond to our carbonyl carbon like that. So I put in my lone pairs of electrons, and there's an R group attached to that carbon as well. So follow our electrons in magenta, functioning as a nucleophile, forming a bond between this carbon and this carbonyl carbon like that. We took a bond away from this carbon down here, so that's a plus 1 formal charge, and so, of course, this is one possible resonance structure. And we could draw a few more possible resonance structures. I'm not going to do that for time reasons. I've done it in the earlier videos, so please watch the earlier videos if you're confused about resonance structures. I'm going to use this resonance structure to represent our sigma complex. And of course, to finish our reaction, we need to deprotonate our sigma complex and regenerate our aromatic ring. So these electrons in here are going to pick up this proton, which would cause these electrons to move in to reform our aromatic ring, and to take away the plus 1 formal charge. And so when we do that, we form our benzene ring with our acyl group now attached to our benzene ring, so it substituted for that proton there. So let's follow those electrons as well. So I'm going to make those electrons in here green. So these electrons in here are going to move into here. And then also we could think about what else is formed. So these electrons up here in red are going to bond to that proton, and so we would also have HCL. So let me go ahead and highlight those electrons in here in red. And then of course, we would also regenerate our catalyst so we would make AlCl3. So we've formed our product. We've installed an acyl group on our benzene ring, and so that is Friedel-Crafts acylation. Let's look at a situation where a Friedel-Crafts acylation might be used instead of a Friedel-Crafts alkylation. So let's say that our goal was to go from benzene to butylbenzene. So let me go ahead and draw butylbenzene out here. So four carbon alkyl group coming off of our benzene ring. So we saw in the last video that a Friedel-Crafts alkylation would make butylbenzene as a minor product because of the rearrangement of the carbocation, and so this would be formed as a minor product. If you wanted to form it in a higher yield, you could use a Friedel-Crafts acylation. And so if I wanted to get to butylbenzene using an acylation, I would need to install an acyl group on my benzene ring that has the same number of carbons. So an acyl group that has four carbons on it. And so let me go ahead and draw that. So we would show an acyl chloride that has a total of four carbons. So there is my acyl chloride, and I can highlight the four carbons. So this carbon, two, three, and four. So to that acyl chloride, we would also need to add our catalyst, right? So we need some aluminum chloride as well. So AlCl3 to catalyze this Friedel-Crafts acylation, and we're going to put this acyl group onto our benzene ring. So here's our acyl group, and you could think about just putting that onto your ring. So when I draw my ring, I know that the carbon on my ring is going to be directly attached to this carbonyl carbon, and there's a total of four carbons here so one, two, three, and then four. And so that's my Friedel-Crafts acylation. And so now I need to go from this compound to my target compound up here, my butylbenzene, and so somehow I need to get rid of that carbonyl. And a reaction that's been historically used to do this is the Clemmensen reduction, which involves a zinc amalgam with mercury, and also a source of proton, so HCL. And the zinc amalgam is going to reduce that carbonyl to our alkyl group so it actually will form the desired alkyl group and gets rid of that carbonyl, and so the Clemmensen reduction is a very useful reaction and synthesis. And so this is a way to make our butylbenzene molecule in high yields using an acylation, which is a little bit more reliable than our alkylation.