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Electrophilic aromatic substitution mechanism

Video transcript

let's look at the general reaction for electrophilic aromatic substitution so we start with a benzene ring and we react benzene with a molecule that contains an electrophile in there and what happens in electrophilic aromatic substitution we're going to substitute the electrophile for a proton on our benzene ring and so over here we can see the electrophile is now in place of that proton so that's where the that's where the electrophilic part comes in this and that's where the substitution part comes in you're substituting an electrophile for a proton the aromatic comes in because you're going to reform an aromatic ring in your mechanism electrophilic aromatic substitution requires a catalyst and the point of a catalyst is to generate your electrophile so down here you can see that the catalyst is going to react to produce the positively charged electrophile so remember electrophile means loving electrons so something is positively charged it's going to love electrons we also form this catalyst complex over here which is going to factor into our mechanism so now that we formed our electrophile let's look in more detail as to what happens in electrophilic aromatic substitution so we start with our benzene ring and I'm showing one of the hydrogen's on the benzene ring it could be any of this of the six since they are all equivalent and now we formed our electrophile from our catalyst so the pi electrons in the benzene ring can be attracted to the positively charged electrophile because negative charges are attracted to positive charges and so pi electrons in your benzene ring are going to function as a nucleophile and and those electrons are going to attack the electrophile so this is a nucleophile electrophile attack where those pi electrons are going to bond to that electrophile there so those pi electrons are going to form a covalent bond with your electrophile so let's go ahead and show that so these PI electrons didn't do anything the hydrogen stays there now I could show the electrophile adding to either of the two carbons on the side of the double bond so it could be that carbon or it could be this carbon since I've drawn this hydrogen up here at the top I'm going to go ahead and say the electrophile adds to top carbon there so there's my electrophile let me go ahead and highlight the electrons that are forming that covalent bond so these PI electrons here are the ones that are functioning as a nucleophile and those PI electrons are going to form this bond right here now in forming that bond we're taking a bond away from this bottom carbon here and so that bottom carbon is going to be left with a positive 1 formal charge therefore we can draw a resonance structure for this cation so let's go ahead and show a possible resonance structure here so these PI electrons could move over to here and let's go ahead and draw what would result if that happens so now we have these PI electrons up here we have our hydrogen we have our electrophile and the electrons moved over to this positions let me go ahead and highlight those in magenta so I'm saying that these PI electrons right here moved over to here and when those electrons move over to there we're taking a bond away from this carbon this time so that is the carbon that's going to get a +1 formal charge like that so we can draw another resonance structure so let's go ahead and do that alright so we could take these pi electrons and move them into here so let's go ahead and show what that would look like so if those PI electrons moved into there we would now have again our hydrogen our electrophile these pi electrons and then these pi electrons right here so once again let me go ahead and highlight those this time I'll use blue so these pi electrons are going to move over to here and once again we're taking a bond away from a carbon this time it's this top carbon up here so that's the carbon that's going to get the +1 formal charge like that so these are all resonance structures and remember the actual cation would be a hybrid of these resonance structures and we call we call that hybrid a sigma complex so you have a positive 1 formal charge delocalized over 3 carbons in your sigma complex alright so the next step in the mechanism I'm just going to redraw the first resonance structure that we did here is I'm going to go ahead and redraw that down here so let's go ahead and show the first resonance structure so in our first resonance structure we had our hydrogen here our electrophile already bonded to our ring and we had a positive 1 formal charge on this carbon right here well remember the catalyst had formed a complexed and I represent it like this so something bonded to your catalyst like that so let's just go up here and refresh our memory alright so right up here when we generated our electrophile alright we also generated this catalyst complex up here so some so Y bonded to a catalyst so I have Y bonded to a catalyst down here and you could think about this as functioning as a base or it's going to accept a proton so I could show these electrons in here taking this proton and if it takes that proton that leaves these electrons behind and those electrons are going to move in here to reform your benzene ring and take away that positive one formal charge so let's go ahead and show that so we now have our benzene ring back and our electrophile is now bonded to our ring and the proton has left so the electrophile has completely substituted for that proton let's follow those electrons again so the electrons in magenta and here so those are the ones that are going to move in here to reform your aromatic ring so deprotonation of the sigma complex restores the aromatic ring and so we have a stable product here so the other product you could think about the this Y here is now going to be bonded to that proton so you could have the wanna here bonded to that proton and you could highlight those electrons you could say that these electrons right here right are now these electrons and taking those electrons away from the catalyst would of course regenerate your catalyst and so it's free - then cattle another reaction and so this is this is the general mechanism for electrophilic aromatic substitution which the reactions that we're going to see are pretty much going to follow this general mechanism