If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:12:58

Video transcript

so far we've looked at electrophilic aromatic substitution reactions involving only benzene but what happens when you start with a substituent already on your benzene ring so we'll look at this molecule over here on the Left which is methoxy benzene so now we have a methoxy substituent on our benzene ring and if we react methoxy benzene with concentrated nitric and sulfuric acids you should recognize that as being a nitration reaction which will install a nitro group onto your aromatic ring the presence of that methoxy substituent is going to affect where that nitro group goes on your ring so for example one of the observed products is to put the nitro group on this carbon which is the carbon right next to the carbon that has our methoxy substituent we say that these two groups are ortho to each other so this would be the ortho product so if i go back over here on this diagram i can label this carbon as being the ortho position on my benzene ring the other product that we observe is nitro group installed on this carbon which is in the opposite side from the carbon containing our methoxy substituent we call this the para product so let me go ahead and label the para position on our benzene ring over here so this would be the para position between the ortho and the para product it turns out that the para product is the is the observed major product in the reaction of the nitration of methoxy benzene and the ortho product is the minor product and this has to do with mostly thinking about steric hindrance and this methoxy group is having some steric hindrance which obviously would prevent this nitro group from adding onto the ortho position easily obviously the para position will have much less steric hindrance and that's the reason that you usually see for the para product being the major product for this reaction now this isn't always the case sometimes your ortho product is more than your para but for this reaction the para product is the major product and once again steric hindrance is one factor to think about when you're doing these kinds of reactions here so there's of course another position on your aromatic ring so if we installed the nitro group on this position we would call this the meta product and the meta product is not observed in high yields for this reaction so we say that the methoxy group is an ortho/para director and we could also label this as being the ortho position on this side and we can say this is the meta position because of symmetry but the meta product is not seen in in a large yield in a high yield and let's go ahead and look at why by drawing a bunch of resonance structures and thinking about the mechanism for electrophilic aromatic substitution and so let's go ahead and start with an ortho attack and so we know that we know that when you're doing a nitration right the sulfuric acid acts as a catalyst to generate the nitronium ion from nitric acid and that functions as the electrophile in your mechanism here so if we're going to do an ortho attack we need to show the nitro group adding on to the ortho position so we need to show the Nitro group adding on to this carbon and so if the Nitro group is going to add on to this carbon then these are the PI electrons that can function as a nucleophile in our mechanism so so we have a nucleophile electrophile reaction right for the first step of our mechanism so the nucleophile alright these PI electrons are going to attack that positively charged nitrogen which kicks these electrons off onto the oxygen so if we draw the result of that nucleophilic attack all right we still have our methoxy substituent up here I'm showing the nitro group adding on to the ortho position and remember there's still a hydrogen attached to that carbon so I have PI electrons over here PI electrons over here and I'm saying that these PI electrons are the ones that formed bond with this nitrogen like that that takes away a bond from this carbon so that carbon gets a +1 formal charge we can show some resonance structures right so we can show some resonance stabilization of this cation here so I could show these PI electrons moving over to here and we could draw another resonance structure so let's go ahead and show the movement of those PI electrons over to this position so let me go ahead and draw in the rest of the ion here so we have a hydrogen here we have an no.2 here and we took these pi electrons right here move them over to this position took a bond away from that carbon so we get a positive 1 formal charge on this carbon and that's another resonance structure we can draw another one all right we can show the movement of these pi electrons into here so let's go ahead and show that we have our ring we have our methoxy group we have once again the Nitro group in the ortho position and we have these PI electrons here and now we showed the movement of those PI electrons over to here so let me go ahead and highlight those these electrons in red move down to here took a bond away from this carbon so that carbon is the one that gets a +1 formal charge now since the oxygen is right next to this carbon right the oxygen has lone pair of electrons and so that lone pair of electrons can give us yet another resonance structure so these electrons could move into here to draw a fourth resonance structure so the presence of that a methoxy substituent right with the lone pair of electrons on that oxygen allows you to draw a fourth resonance structure so this would give this auction a plus 1 formal charge all right we have these PI electrons over here we have our nitro group once again in the ortho position and let me just go ahead and show the movement of those electrons so these electrons I'll make them green these electrons right here are going to move in to form our PI bond like that and we have a total of 4 possible resonance structures remember remember the ion is actually a hybrid of these four resonance structures which we call our sigma complex and so again the presence of this methoxy substituent right with a lone pair of electrons right next to our aromatic ring gives us an extra resonance structure this one is our extra resonance structure and so we have a total of four resonance structures for an ortho attack and the more resin structures you can draw the more that positive charge is delocalized lies in the more stable your Sigma complex is the more stable your Sigma complex is the more likely it is to form in your mechanism for electrophilic aromatic substitution and so because we can draw for resonance structures for an ortho attack that is a favored as a favored carbo cation that's a stable sigma complex and that's going to form much more easily and of course the last step in your mechanism is to deprotonate your sigma complex to reform your aromatic ring but i'm not going to show that step here I just wanted you to see the four possible resonance structures for an ortho attack that's different from a meta attack so let's go ahead and look at what would happen alright if we added on our nitro group meta so the meta position would be of course this one right here so we would use these pi electrons right so nucleophilic attack and kicks these electrons off onto our oxygen so let's go ahead and show the result of our nucleophilic attack and adding on our nitro group in the meta position so if we're going to show the nitro group in the meta position right like that let's highlight these electrons here so these PI electrons are going to come off on to and form a bond I should say with that nitrogen taking a bond away from that top carbon so that top carbon is going to get a +1 formal charge and of course we have our other PI electrons in our ring like that so resonance structure right I could draw a resonance structure for this ion here I could take these PI electrons move them over to you here so we'll go ahead and show another resonance structure so I have those PI electrons moving over to there I had these PI electrons here I still have my nitro group in the meta position to my original methoxy substituent like that and now my positive one formal charge is on this carbon right here so to save time I'm not going to color coordinate these resonance structures I can draw one more right I could show these pi electrons moving over to here and let's go ahead and draw that resonance structure so I have those PI electrons move over here I have these pi electrons I have my methoxy substituent I have once again my nitro group and of course now my positive charge moves down to this carbon down here and that's it I can only draw three resonance structures so once again the actual Sigma complex is a hybrid of these three resonance structures and since I can only draw a three right only a total of three resonance structures for this situation this Sigma complex is not as stable as the one that we saw for an ortho attack so the Sigma complex for an ortho attack was more stable because we could draw four resonance structures and here we can only draw three let's go ahead and show a para attack so if I want to show my nitro group adding on to the para position all right so here is the para position so I'm going to use I'm going to use these PI electrons right so nucleophilic attack alright kicks these electrons off so this would be our para attack adding on our nitro group into the in the para position so once again I have my methoxy group so this time I'm going to show write my nitro group in the para position so let's go ahead and follow these PI electrons so these PI electrons in here alright those PI electrons formed a bond with that nitrogen and I took a bond away from this carbon so that is where my plus 1 formal charge is going to be I still have these PI electrons in my ring like that so a resonance structure for this one I could show these electrons in here moving over to there and let's see what we would get we would still have once again this substituent right our PI electrons moved over to here these PI electrons were here I had my nitro group para and now I took a bond away from this carbon right so that is the carbon that gets the +1 formal charge so I can draw another resonance structure right I could show I could show these electrons in here moving to there so let's get a little more room here for more resonance structures so I can show my ring and I can show these PI electrons have moved over to here I still have my substituent right and of course my nitro group is still in the para position I took a bond away from this carbon so I get a plus 1 formal charge and once again the presence of that lone pair of electrons on that oxygen right next to my benzene ring allows me to draw another resonance structure so I could think about these electrons on my oxygen moving into here to form a PI bond right and pushing these electrons down to here and so I can draw a fourth resonance structure showing the oxygen now double bonded to my ring alright so there's still a lone pair of electrons on that oxygen which will give it a plus 1 formal charge I still have PI electrons here I showed PI electrons moving over to here and once again my Nitro group is still in the para position and so I have a total of four resonance structures if I if I show the nitro group adding on in a pair of fashion as well so so the para the para attack also has four resonance structures and the ortho had four resonance structures and so that helps to explain why the methoxy substituent functions as an ortho para director if you show ortho ortho or a para attack you can draw a total of four resonance structures which stabilizes the sigma complex more than a meta attack and that's the reason for the regio chemistry that we see in the nitration of methoxy benzene now I drew these resonance structures because I started with my with my benzene ring right I started with my electrons in in this position right here now if you started with a different benzene ring right so a resonance structure of that then your resonance structures might look a little bit different from mine and you'll see different versions in different textbooks so make sure to once again always look at what your professor does in class or your textbook and think about that on exams if they have you draw resonance structures for an ortho para or a meta attack