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Current time:0:00Total duration:12:56

Video transcript

in the last video I promised you that I would show a concrete example of electrophilic aromatic substitution so let's do that right here so let's say we have some benzene we've got some benzene we've got some benzene it's a solution with some molecular bromine so I'll draw the molecular bromine like this so it's one bromine right there it has one two three four five six seven valence electrons and it's bonded to another bromine that has one two three four five six seven electrons and these two guys in the middle are bonded to each other they can kind of act as an electron pair or make bro means feel like they have eight electrons this guy thinks he has a magenta one that guy thinks he has the blue one and let's say we have some iron bromide in our mix as well and we're going to see is that this is going to catalyze the reaction so the iron bromide we have some iron it's bonded to three bro means it's bonded to three bro means just like that so we haven't seen iron bromide much but the way I think about it is that bromine molecules are much more electronegative than the iron so even though these look like fair covalent bonds if we thought about oxidation these guys are going to be hogging the electrons this actually would have an oxidation number of three and in reality they are hogging the electrons they are more electronegative so the way I think about it is that this iron will have a slightly positive charge because the electrons are being hogged away from it from it so it wouldn't mind to gain an electron or it might want to accept an electron it might want to act as a Lewis acid remember the Lewis acid will accept an electron so this might want to act as a Lewis as a lewis acid so who can it NAB electrons from well maybe get an AB an electron from this bromine right here and you know I'm not saying that this is always going to occur but under the right circumstances if they bump into each other with the right energies it can happen so this electron let me do it in a different color that I haven't used yet this green color let's say this electron right here gets nabbed by that iron then what do we have well then we have a situation we have this bromine the blue one with one two three four five six seven valence electrons we have the magenta bromine with with one two three four five now it only has the sixth valence electron right here the seventh got nabbed by the iron so the iron has the seventh valence electron and then you have the rest of the molecule so then you have your iron you have your iron and it's attached of course to the three bro means to the three to the three bro means just like that and then our bonds these guys were bonded they still are bonded and now these guys are bonded these were in a pair this electron jumped over to the iron now we have another Bond but because this bromine lost an electron it was neutral it lost an electron it now has a positive charge it now has a positive charge and the iron now that it gained this electron now has a negative charge it now has a negative charge so let's think about what's going to happen now now we're going to get the we're going to bring the benzene into the mix so let me redraw the benzene let me redraw the benzene just like that and we have this double bond that double bond and then just to make things clear let me draw this double bond with the two electrons on either end so we have an orange electron you have your green electron right over there and I'll draw the double bond as being green now let's think about this molecule right here we have a bromine with a positive charge bromine czar really really a really electronegative you might see them with a negative charge with a positive charge it really wants to grab an electron and in the right circumstances you can imagine where it really wants to grab it really really really really wants to grab that electron right there so maybe if there's just some way it could pull this electron but the only way it could pull this electron is maybe if this because if it just took that electron then this bromine would have a positive charge which is cool so this bromine maybe would want to pull an electron if this guy gets an electron then this guy can get an electron so you can imagine this thing this thing as a whole really really wants to grab an electron it might be very good at doing it so this is our strong electrophile so what actually will happen in the bromination of this benzene ring and let me draw some hydrogen's here just to make just to make things clear we already have we already have hydrogen's on all of these carbons sometimes important to visualize this when we're doing electrophilic aromatic substitutions so we already have a hydrogen on all of these molecules so maybe this is so electrophilic it can actually break the aromatic ring nab this electron right there so maybe this electron right there this electron right there goes to the bromine goes to there maybe I should even drew it this way just so you make it clear kind of replaces that one although it you know obviously the electrons are a bit fungible so maybe it goes over there and then when it goes to this let me make it clear it's going to the blue bromine so this electron right here goes to the blue bromine if that if the blue bromine gets an electron then it can let go of this blue electron so this blue electron can then go to this bromine right over here and then what is our situation look like well if we have that then let me draw our benzene ring first we have our benzene ring first let me draw the benzene ring benzene ring this double bond that double bond let me draw all of the hydrogen's one hydrogen one duman's purple one atom in purple so we have one hydrogen two hydrogen's three hydrogen's four hydrogen's five hydrogen's and six hydrogen's this orange electron is still with this carbon right here so this orange electron is still with this carbon right here but that electron got nabbed by this bromine so that electron got nabbed by this bromine right over here by that bromine I've kind of flipped it around and now it has its other six valence electrons two three four five six the electron got taken away from this carbon so now that carbon will have a positive charge but we saw in the last video it's actually resonance stabilized that electron could jump there that electron can jump there so it's not as stable as a nice nice aromatic benzene ring like this but it's not a ridiculous carbo cation it's stable enough for it to exist for some small amount of time while we kind of hit our transition state and then this molecule over here what's it going to look like what's it going to look like well this bromine had a positive charge it had a positive charge now it gained an electron let me draw it so you have you have your bromine you have your bromine it gained an electron so now it is neutral again so let's see it had the one one two three four five six now gain this blue electron so now seven valence electrons back to being neutral it's bonded to the iron bromide bonded to the iron bromide we draw the bro means one two three and so we've given this for this blue bromine to the benzene ring but it's not happy here it doesn't want to break its aromatisse ax t it wants an electron back so how can it gain an electron back well this thing actually let me make it very clear let me make it clear this thing had a negative charge this thing had a negative charge right here so you can imagine send this we had this electron right here so maybe this thing right over here can now act as an actual base it can nab a proton off of the benzene ring just like we saw in the last video this is now the base this whole complex to some degree acted as a strong electrophile now that we got one rid of one of these bro means this thing might want to grab a proton now since it is positive and will act as our base and will nab one of these protons if it NAB's the proton then the leftover electron is still there that electron is still there let me do that front color that electron is still dude in red that electron is still there and then that electron can be given to that original carbo-cation and we'll have a nice aromatic ring again so how would that look so you could imagine you could imagine a situation where this electron this green electron right here gets given given to the hydrogen nucleus if it's given to that hydrogen nucleus then this red electron right here can then be given to the carbo-cation and then what are we left with what are we left with well we have our we draw what we started with so we have our ring we have this double bond and that double bond right there now let me draw all of our hydrogen's we have this hydrogen that hydrogen this one over here this one over here and we have this one over here now this hydrogen just now got nabbed so this hydrogen over here got nabbed it got given actually the hydrogen nucleus got given this green electron it got given this green electron which is paired with this magenta one right over here so it's paired with this magenta one right over here so it is now bonded to the bromine we now have hydrogen bromide and this had one two three four five six other valence electrons I'll keep I'll keep the colors consistent and this is now bonded now this electron went away from the iron so the iron will now lose its negative charge the iron bromide so now it is back in its original form it is now back in its original form so we have our iron bonded to three bro means we are bonded to three bro means just like that it has lost its negative charge and this this electron right here has now gone to the carbo cation so that electron right there has now gone to the carbo cation it is right there so this bond you can imagine is now part you can imagine it now being this double bond so it was magenta so I'll draw it in magenta just there and we can't forget the whole point of this whole video was the bromination so now we now have this bromo group there so we have this orange electron is right over here and it is bonded to it is bonded to that bromine it is bonded to that bromine just like that and we have brominated the benzene ring using and just let's be clear we had a negative charge and a positive charge now it's cancelled out the thing with the negative charge gave an electron to the positive charge it is now neutral that is now neutral now let's be clear and what happened here we want to just to map it to what we saw before we had a benzene ring right here we have no strong electrophile or strong base yet it had to become a strong electrophile when this thing gave an electron to the iron bromine became this larger molecule now this whole thing can act as a pretty strong electrophile this grabbed an electron it broke though it broke the aromatisse 'ti of the benzene ring but just long enough long enough for this bromine to form but once this bromine it gained an electron and then and bonded to the benzene gave and then gave up an electron to this other bromine that really wanted to get an electron because it had a positive charge and then once it got it now this whole thing acted as the base so we kind of have the same molecule changing up a little bit acting as an electrophile or acting as a base and then once it acts as a base this bromine this magenta bromine NAB's the proton allows this electron to go back to that carbo cation and then we're left with the iron broma bromide again so this thing really didn't change through the whole reaction that's why we can call it a catalyst it wasn't one of the reagents one of the reactants in the reaction hopefully you found that vaguely interesting