Contenu principal
Heure actuelle :0:00Durée totale :11:17

Transcription de la vidéo

we've already talked about how a benzene ring is very let me draw a better looking benzene ring than that that a benzene ring is very stable because it's aromatic that these electrons in these PI orbitals that form these double bonds they're actually just not in this double bond they can keep swapping this one can go here this one can go there that one can go there actually they don't go back and forth they actually just completely go around the entire the entire ring and when a molecule is aromatic it stabilizes it but we've seen examples of aromatic or actually in particular we've seen examples of benzene rings that have other things bumping off of them whether they're halides or whether they're o-h groups and what we want to do in this video is think about how that might happen how do things get added on to a benzene ring so we're going to learn about electrophilic aromatic substitution let me write that down electro electro Cilic electrophilic aromatic aromatic substitution substitution and you might say well Sal you just said you're adding things to the ring but the reality is is that there's six hydrogen's here there's one hydrogen two hydrogen's three hydrogen's four hydrogen's five hydrogen's and six hydrogen's they're always there if you don't draw them they are implicitly there so we're actually doing when you add a when you add a a chlorine or bromine or a no age group it's actually replacing one of these hydrogen's that's why it is substitution it's aromatic because we're dealing with benzene ring we're dealing with an aromatic molecule and we're going to see that we need a really strong electrophile in order to do this so let's think about this how this will happen before I do that let me just copy and paste this because I don't want to have to redraw this so let me just copy it just like that so let's say we have a really strong electrophile and i'll give you particular cases in the next few videos so you can better visualize what a really strong electrophile is but just from the word itself electrophile you could imagine it's something that loves electrons it wants electron really really really really really badly and usually it has a positive charge usually has a positive charge so it wants electrons badly wants electrons actually let me make it very clear instead of saying wants electrons badly because when you're talking about electro files or nucleophiles you're actually talking about how good something is reacting you're not actually talking about the actual energies involved so let me put it a different way good at getting electrons good at getting electrons really really really really really good at getting electrons so what would happen what we already said this is already pretty stable these guys that can these electrons these PI electrons can circulate all around but if it's if we if it gets if it bumps just in the right way to something that's really good at getting electrons what might happen what might happen let's say we have this this electron right here that the way we've drawn it it's on this carbon right here obviously the carbon is just at the intersection I never drew the carbon but if this electrophile which is really good at getting electrons bumps in just the right way this electron can go to that electrophile that electron can go to that electrophile and then would be left with so let me copy and paste our original molecule so then what would we be left with so we no longer we no longer have we no longer have this bond right here we no longer have this bond right here it has now been bonded to the electrophile let me make it clear we had this electron right here that electron is still on this carbon right over here at this intersection but the other end of it the other electron has now been given to the electrophile but think that lot that's good at getting them so the other side has been given to this electrophile thus electrophile now gained an electron so it had a positive charge now it will be neutral and once again I'll show you a particular or several particular cases of this in the next few videos and let me just make it clear so this bond this bond you can now view it as being this bond now this carbon right over here this lost an electron so if it lost an electron it will now have a positive charge it will now have a positive charge now this is hard to do to a resonance stabilized molecule to a benzene ring so that once again I said I'm being a little bit repetitive this has to be a very good electrophile to do it but once this is there this is actually a relatively stable carbo cation and the reason why it is it's only a secondary carbo cation but it's actually a resident stabilized carbo cation because this guy can go this electron right here can be given to that so this if this electron goes there then it would look like this let me redraw it I'll draw the resonance structures quickly so you have your hydrogen you have your electrophile that's what's not electrophile anymore but you have that II that's now been added you have that hydrogen you have a double bond here let me make that draw a little bit neater you have this hydrogen you have this hydrogen this hydrogen and this hydrogen and what I said is this is stabilized so when electron here can actually jump over here and then so if this electron jumps over here the double bond is now over there if that goes over there like that the double bond is now over here and now this guy now this guy lost this electron it would have a positive charge and then that is resonance stabilized it can either go back to this guy or or this electron over here can jump over there so let me redraw the whole thing over again redraw the whole thing let me draw all the hydrogen's so this right here you have the E and the hydrogen you have a hydrogen here hydrogen here hydrogen here hydrogen here and normally you don't worry about the hydrogen's but one of the hydrogen's is going to be nabbed later on in this in this mechanism so I want to draw the hydrogen's so you know that they are there but as I said this is resonance stabilized if if this electron right here jumps over there then this double bond is now this double bond and now this guy over here lost an electron so it would have positive charge and again once you had this double bond up here this double bond up there's that double bond so we can go back and forth between these electrons or just swishing around the ring so it's not going to be maybe as great as the situation that we had when we had a nice benzene ring that was completely aromatic the electrons could just go around the p-orbitals round and round the ring stabilize the structure but this is still a relatively stable carbo cation because the electrons can move around you can kind of view it as a positive charge it gets dispersed between this carbon this carbon and that carbon over there now this as I said this is still not a great situation it wants to get the molecule wants to go back to being aromatic wants to go to that really stable state and the way it can go back to that really stable status somehow an electron can be added to this thing and the way that an electron can be added to this thing is if we have some base flying around if we have some base flying around and that base NAB's this proton this proton right here that it's on the same carbon as where the electrophile is attached so this base NAB's a proton so it just snaps the hydrogen nucleus then that electron that the hydrogen had that electron let me do that in a different color i reused that electron that the hydrogen had right over there could then be returned to this carbon up there and maybe that makes a little confusing when I cross lines I could be returned to that carbon right there so what would it look like after that after that it would look like this so let me draw my so if that happened we do it in yellow we have our six carbon ring six carbon ring let me draw all the hydrogen's so what did I do that in it looks like a slightly green color I did that in so I have all the hydrogen's on that ring all of the hydrogen's now have to be careful this hydrogen right there just the nucleus of it got nabbed by this base so that hydrogen has now been nabbed by the base if you view this electron right here has now been given to this hydrogen so that electron has now been given to this hydrogen and then the other electron in the pair still with the base the other electron is still with the base so now this is the conjugate acid of the base it has gained a proton and on this carbon right here we just have what was the electrophile we just have what was the electrophile and i'll do the same colors just to make it clear what was the electrophile right over there this bond this bond is or this bond is this bond and then finally we had an all color-coded here just made clear we had this double bond here which is this double bond right over here we had this double bond we had this double bond which is that double bond there and this electron and was this electron gets returned to this top carbon right here so that electron that electron let me make it very very clear so the bond and that electron are returned to that top carbon so that we have the bond and that electron returned to that top carbon that top carbon is now is now going to be neutral and once again we are resonance stabilized on one thing I forgot just to make the charge stabilized maybe this base had a negative charge to begin with it didn't have to but if this base did have a negative charge to begin with it now gave an electron it gave an electron to the hydrogen so it is now neutral and this should make sense because before we had a plus charge and a negative charge and then when everything reacted everything is neutral again the total net charge is zero but this is the electrophilic aromatic substitution we substituted we substituted one of the hydrogen's we substituted this hydrogen right here with this electrophile or what was previously the electrophile but then once it got an electron we just it's just kind of a group that is now on the benzene ring and by going through this little convoluted process we finally got to another aromatic molecule that now had this now that has this egroup on it in the next video I'll show you this with particular examples of electro files and bases