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Current time:0:00Total duration:8:23

Video transcript

here's the general reaction for the nitration of benzene so we start off with benzene and to it you add concentrated nitric and concentrated sulfuric acids and that puts a nitro group onto your benzene ring in place of this proton let's look at the mechanism for the nitration of benzene so we start over here with the dot structure for nitric acid and here is the dot structure for sulfuric acid and sulfuric acid is actually a stronger acid than nitric acid so the first step is sulfuric acid is going to function as a bronsted-lowry acid and donate a proton and so I'm going to say it's this proton right here and nitric acid is actually going to function as a base and accept that proton so we can go ahead and show this lone pair of electrons picking up this proton and these two electrons in here remaining behind on that oxygen so let's go ahead and show that acid-base reaction so we would have our compound over here so let's go ahead and draw on these atoms alright so this oxygen is still going to have a negative 1 formal charge on it and then over here on the right this oxygen already had one hydrogen bonded to it just picked up another proton all right so it still has one lone pair of electrons which gives that a +1 formal charge so let me go ahead and highlight these electrons here so I'm saying that this lone pair of electrons right here all right picked up a proton right like that which gives that oxygen a plus 1 formal charge and notice that forms water as a leaving group and so if these electrons were to move in here right that would kick these electrons in here off onto the oxygen so let's go ahead and draw the result of that so we would have water that left right so let's go ahead and show h2o over here on the right so go ahead and draw on those electrons like that so I'm saying that these electrons in here in magenta are the ones that came off on to the water molecules of water as our leaving group and if water leaves that leaves what's called the nitronium ion behind so let me go ahead and draw the nitronium ion here so it looks like this now let me go ahead and highlight those electrons so I'm saying that these electrons in here right moved in to form a PI bond so I'm saying it could be represented by those electrons down here on my structure now for formal charges this nitrogen has a plus 1 formal charge and that nitrogen is actually SP hybridized which makes the nitronium ion linear and the nitronium ion is positively charged this is going to be the electrophile in our mechanism right so this is our electrophile our positively charged ion so up here if we think about it we would also create the conjugate base to sulfuric acid so hso4 minus would be over here alright so let's go ahead and and show what happens now that we've formed our electrophile so the electrophile is going to add to the benzene ring so we're gonna have a nucleophile electrophile reaction so let's go ahead and draw our benzene ring over here and I draw in my hydrogen on my benzene ring we've now formed our nitronium ion right so the point of the catalyst was to produce our electrophile here so we have a positively charged nitronium ion like that and so now we have a nucleophile electrophile situation where once again the pile Ektron is in our benzene ring are going to function as a nucleophile and attack our electrophile so those electrons are going to attack that positively charged nitrogen which you kick these electrons in here off onto that oxygen so let's go ahead and draw the result of our nucleophilic attack so we have our ring right here we have a hydrogen and once again just for convention sake I'm going to show our electrophile adding to that top carbon of the double bond there of what used to be the double bond and so now we have a nitrogen there we have a nitrogen double bonded to our top oxygen and then over here we would have an oxygen right with three lone pairs of electrons giving that a negative 1 formal charge and the nitrogen of course is still going to have a +1 formal charge like that alright let me go ahead and highlight those electrons right so once again these PI electrons are going to be attracted to the positive charge right nucleophile electrophile and those PI electrons are going to form this bond right here to our nitro group all right well once again as we've seen several times before we took away a bond from this carbon right so that's where our plus 1 formal charge is going to go like that and so we can draw we can draw some resonance structures right so let's go ahead and show a resonance structure for this so we could move these pi electrons in over here so let's go ahead and draw that so we had a hydrogen up here and you could just show it a nitro group as no.2 so I'm just going to go ahead and do that to save some time these PI electrons over here alright are still there and I'm saying that those PI electrons moved over to here so let me go and highlight those so these PI electrons in blue alright move over to here took a bond away from that carbon so now we can put a plus 1 formal charge right at that carbon like that we can draw yet another resonance structure so I could show these electrons over here moving to here so let me go ahead and draw that alright so we have our ring we have our nitro group already on our ring we have some pi electrons right here and we have some more PI electrons moving from here to here which of course takes a bond away from this top carbon so that's where our positive 1 formal charge is now so now we have our our three resonance structures and remember once again that sigma complex is a hybrid of these three and we're now ready for our last step right so deprotonation of our sigma complex so if we go back up to here we think what could function as a base well the water molecule here could function as a base so a lone pair of electrons on our water molecule are going to take that proton alright which would cause these electrons to move in here to reform your aromatic ring so let's go ahead and show that so we're going to reform our benzene ring here and we took off the proton right so deprotonation of the sigma complex yields our product with a nitro group substituted in so let me go ahead and and highlight those electrons again so this time I'll use green so these electrons right in here right when that Sigma complex is deprotonated those electrons are going to move in here to restore the aromatic ring and we have created our product we have added in our nitro group so that's the mechanism for nitration now once you once you once you form a benzene ring with a nitro group on it sometimes when you're doing synthesis it's helpful to turn that nitro group into an amine so let's go ahead and real quickly look at another useful reaction here so once you form once you form your your nitro group like that you can turn that nitro group into a mean into an amine a couple of different ways so one of the classic ways to reduce a nitro group would be in the first step to use either iron or tin alright and source of protons so something like HCl will work well and since you're doing this in an acidic environment and the second step you would need to neutralize it was something like sodium hydroxide and those steps will reduce the nitro group to an amine so let me go ahead and show the product here so instead of an no.2 on the ring now you have an NH 2 so of course this would be the aniline molecule and there are other ways to to reduce a nitro group to an amine um this is just this is just one of the classic ways to do it so it could be useful for synthesis problems which we will study later in this tutorial