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Current time:0:00Total duration:9:12

Video transcript

in the last video we looked at our friedel-crafts alkylation and this video we're going to do an isolation which is very similar to the alkylation we start off with benzene and two benzene we add an ACL 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 installation 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 I'm not going to draw 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 right 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 on to 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 the 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 bond to this top oxygen here this top oxygen would still have a lone pair of electrons and have a +1 formal charge like that and this carbon is still bonded to an R group so I I'm 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 isolation and alkylation because our cation in an isolation is resonance stabilized there is no rearrangement and that's different from what we saw in our alkylation reaction we formed a carbo cation 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 isolation 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 all right 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 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 +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 right so once again negative charges attract positive charges these PI electrons are going to function as a nucleophile attack our electrophile and so we can add our electrophile on to our band 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 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 video so please watch the earlier videos if you're confused about resonance structures I'm going to use this resin 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 +1 formal charge and so when we do that right we form our benzene ring with our acyl group now attached to our benzene ring so it's 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 right 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'd also regenerate our catalyst so we would make alcl3 so we've we've formed we formed our product we've installed an acyl group on our benzene ring and so that is friedel-crafts isolation let's look at a situation where a friedel-crafts isolation might be used and a friedel-crafts alkylations so let's say that our goal was to go from benzene to butyl benzenes let me go ahead and draw butyl benzene 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 butyl benzene in a minor as a minor product because of the rearrangement of the carbo cation 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 isolation and so if I wanted to - to get to butyl benzene using an isolation 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 of this carbon two three and four so two that acyl chloride we would also need to add our catalyst right so we need some aluminum chloride as well so alcl3 two catalyzes friedel-crafts isolation and we're going to put this acyl group onto our benzene ring all right so here's ours 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 isolation and so now I need to go from this compound to my target compound up here my butyl benzene 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 so it actually will form on the desired alkyl and gets rid of that carbonyl and so the Clemmensen reduction is a very useful reaction in synthesis and so this is a way to to make our butyl benzene molecule in high-yield using an isolation which is a little bit more reliable than our alkylation