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Current time:0:00Total duration:8:40

Video transcript

this video we're going to look at the halogenation of benzene and we'll start with bromination so here's a benzene ring and - it's we're going to add some bromine and our catalysts will be aluminum bromide and you could have used febr3 instead of instead of al b r3 that's fine and the end result is substitution of a bromine atom for an aromatic proton on your ring let's look at the mechanism for this electrophilic aromatic substitution reaction and if I look at the aluminum bromide catalyst and I can see there are six electrons around the aluminum atom so here's two and four and then a total of six and so because of because of aluminum's position on the periodic table it can actually accept two more electrons so the aluminum bromide is going to function as an electron pair acceptor which is the definition for a Lewis acid the bromine is going to function as a Lewis base it's going to be an electron pair donor so we could think about this lone pair of electrons in here being donated to the aluminum and a bond forming between that bromine and that aluminum so let's go ahead and draw the result of that lewis acid-base reaction and so now this bromine is bonded to this aluminum this aluminum is still bonded to these other bro means here I'm not going to draw on those lone pairs electrons around those bro means just to save some time let's follow those electrons so the electrons in magenta those are the ones that were donated it to the aluminum and forming this bond between the bromine and the aluminum that would give the aluminum a negative 1 formal charge and this bromine would get a +1 formal charge like that now technically this is the complex it's going to react with our benzene ring in our mechanism but it's kind of hard to see the electrophile in this complex so let me just go ahead and show you what you can think about the electrophile being and then we'll come back to this complex in the mechanism with benzene so if these electrons in here moved off onto the bromine on the right the bromine on the left will have lost a bond so it would now only be surrounded by three lone pairs of electrons giving it a +1 formal charge and it simplifies things to think about this as being the electrophile in your mechanism for electrophilic aromatic substitution even though technically it's going to be this top complex here that's going to react with our benzene ring so if we form BR plus over here on the right we would have this bromine now still bonded to this aluminum and it would have three lone pairs of electrons around it now so let me go ahead and highlight these electrons in here in red I'm saying they're going to kick off onto that bromine there like that and the aluminum of course is still bonded to these three other bromine and it still has a negative one formal charge like that so we've generated our electrophile or this is one way of simplifying it to think about the BR plus as being our electrophile and so that could react with our benzene ring so we come back to our benzene ring here and we think about BR plus as being the electrophile and so remember electrophile means loving electrons and so it's it is attracted to electrons and of course electrons are negatively charged and they're attracted to positive things so you could think about these pi electrons in your benzene ring is functioning as a nucleophile and so we get a nucleophile attacking an electrophile here and so let's go ahead and show the result of that nucleophilic attack so we have our ring we have our PI electrons in our ring and you could show the bromine adding to either one of these carbons it doesn't really matter since they are equivalent I'm going to show the bromine adding to the top carbon there so the top carbon already has a hydrogen on it and we're going to say that these electrons in here add on to the bromine so let me go and highlight the electrons that we're talking about so these PI electrons in here function as a nucleophile alright form a bond with that bromine like that we took a bond away from this carbon down here so we're actually going to get a +1 formal charge at that carbon so we make a carbo cation now remember technically it's actually it's actually this complex up here that's reacting with benzene so we could think about a better Mac or a more accurate mechanism as being these electrons in magenta going up to here attacking that bromine and then the electrons in red here once again kicking off onto this bromine forming this complex over here and so that's the more accurate way of thinking about it for me it just simplifies things to think about BR plus as being an electrophile so now that we formed a carbo cation here we can actually draw some resonance structures and so if I took these pi electrons and moved them into here let's go ahead and draw the resulting resonance structure for that so I would have my ring I would have these pi electrons I would have a hydrogen and a bromine still attached to my array and I would move those PI electrons over to here so let me go ahead and highlight those so these PI electrons moved over to here took a bond away from this carbon this time and so that's the one that's going to get a +1 formal charge like that we could draw another resonance structure so these PI electrons up here could move down there and let's go ahead and show the result of that so once again we have our ring we have a hydrogen a bromine these pi electrons are still there and we get some pi electrons moving over to here so let me highlight those now so these PI electrons right here move over to here took a bond away from this top carbon and so that top carbon is going to get a plus 1 formal charge and so we have three resonance structures that we could draw for this carbo cation and remember the the actual ion is a hybrid of our three resonance structures and we could think about that hybrid as being a sigma complex so in electrophilic aromatic substitution the last step of the mechanism is deprotonation of your sigma complex to reform your aromatic ring and so we could think about we could think about this complex right here it's going to function as a base and I'm going to say that these electrons in here alright I could take this proton and then these electrons would move into you hear to reform our aromatic ring so let me go ahead and draw the product which is bromo benzene and since we have a lot of arrows going on there let me go ahead and highlight some electrons so we can follow them so let me go ahead and make these electrons in here blue so these electrons are going to move in here to reform our aromatic ring and these electrons in here now you could think about this complex as functioning as a base and so those electrons pick up that proton and so you'd form HBR as another product so we'd have HBR here and I'll highlight those electrons in green like that and then of course we would also reform our catalyst so al BL 3 our catalyst has been reformed in this reaction and so that's that's the mechanism for the bromination of benzene if you want to think about adding other halogens onto your benzene ring let's go ahead and look at chlorination here so if we started with a benzene ring and we wanted to put a chlorine on our benzene ring we would add some CL 2 and our catalyst would be you could use alcl3 aluminum chloride or you could use fecl3 it doesn't really matter which catalysts that you use here the end result of course would be to substitute in a chlorine for one of the protons one of the aromatic protons on your ring here and so our our product our product would be you see if I can draw this in here so we would have our benzene ring and we would have a chlorine on our benzene ring to form chlorobenzene and if it helps you to think about HCl as being another product formed in this reaction it is and so these are these are just two two mechanisms for the halogenation of benzene