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Bromination of benzene

The bromination of benzene is an example of an electrophilic aromatic substitution reaction. In this reaction, the electrophile (bromine) forms a sigma bond to the benzene ring, yielding an intermediate. Then, a proton is removed from the intermediate to form a substituted benzene ring. Created by Sal Khan.

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  • leaf green style avatar for user mw7890
    Would this be considered a biological chemical reaction?
    (3 votes)
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    • spunky sam blue style avatar for user Ernest Zinck
      No. The body does not use FeBr₃ and bromine to form bromobenzene. Bromine is corrosive and poisonous..Benzene is toxic and can cause leukemia-like diseases and genetic defects.
      Benzene is metabolized, primarily in the liver, to a variety of OH-containing and ring-opened products that are transported to the bone marrow, where further reactions occur. Benzene is BAD STUFF .
      Most biological chemical reactions, including the metabolism of benzene, are controlled by enzymes. Many of these enzymes do have metal ions such as Fe³⁺ in their active sites.
      (8 votes)
  • leaf green style avatar for user hunae
    at the Hydrogen gets an extra electron so it can give it's original one to the unstable carbocation. Why does the hydrogen leave the benzene if it would be stable in a situation where it gave it's electron away and got one back 1-1+1=0 neutral? Is the reason that once hydrogen gives an electron away there's energy/heat released so that the C-H bond automaticly cuts off and you would actually need additional heat to the system in order to maintain/reform that bond?
    (5 votes)
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    • blobby green style avatar for user Hernando Steven Castillo
      There are two things you have to remember. First, the only reason it can give up the first electron is because it is being "given" one by the Fe but that electron it is giving is already in a connection to with the 4th Br on the Fe. Fe can't exactly just hand over that electron. That electron is coming with the Br.

      Second, If the H were to give its electron to the Carbon adjacent to it, and then with the electron it received just stay at the Carbon its currently located at, you would have that Carbon have a bond to hydrogen, a bond to bromine, a double bond to one carbon adjacent to it, and another bond to the other carbon adjacent to it; for a total of 5 bonds. Carbon can not make 5 bonds
      (3 votes)
  • blobby green style avatar for user Pratik Agrawal
    you told that br really in need of electron as it is too electrophilic ,can't it take back the electron which it has given to febr3 compound? Because it is already bonded to fe compound........i mean it would be easier for br to take that electron rather taking it from a stabilized benzene ring!
    (5 votes)
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  • male robot hal style avatar for user Aakash Rajesh
    At , If Br is so electronegative, why does it give it's electron to Fe in the first place?
    (2 votes)
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  • leaf green style avatar for user Daniel
    hey, i noticed that there isn't a video about chlorination of Benzene, why?
    (2 votes)
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  • leaf green style avatar for user darsh.shah.51200
    @ isn't dipole movement 0 due to sp2 hybridization so then shouldn't Fe have o charge? then y does sal say it will have charge?
    (1 vote)
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    • spunky sam blue style avatar for user Ernest Zinck
      He is talking about electronegativity and oxidation numbers, not dipole moment or hybridization.
      Br is more electronegative than Fe, so Br will have a δ⁻ charge and Fe will have a δ+ charge.
      The oxidation numbers give the extreme case, where the oxidation number of Br is -1 and the oxidation number of Fe is +3.
      No matter how you look at it, Fe will have a partial positive charge and will act as a Lewis acid.
      (5 votes)
  • mr pink orange style avatar for user Aarushi Gupta
    At , why does the bromine have a +ve charge ? It has got 8 electrons ,hasn't it ?
    Bromine has a +ve charge and iron has a -ve charge ,this results in the bond formation . shouldn't the charges disappear after the bond is formed between iron and bromine ?
    (2 votes)
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    • starky ultimate style avatar for user Swarna
      Wow. I hadn't noticed that before. Sal says that that Bromine which was neutral lost an electron (to form the bond). Hence, it gains the positive charge. Yes, when you look at it as a whole you might wonder that why does it even have a positive charge because its octet is complete. Did you notice that FeBr3 has a negative charge? [As he said it is very electronegative (though that shouldn't really make a difference I guess) AND it has gained this electron from Bromine)]. So when you look at it with the negative charge it might make much more sense!
      (2 votes)
  • blobby green style avatar for user Anvesh K R Dasari
    why not FeBr3 reacts with benzene? FeBr3 is also an electrophile right? When FeBr3 can get electrons from Br2 why can't it get it from Benzene?
    (1 vote)
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  • orange juice squid orange style avatar for user Iyad Bizmawi
    I don't understand why the magenta Bromine got a positive charge despite the fact it got a full octet at . I don't, also, get why the Iron got a negative charge.
    (2 votes)
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  • leaf orange style avatar for user Lohit Gandham
    What is the difference between an electrophile and a Lewis acid?
    (1 vote)
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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. And 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. These two guys in the middle are bonded to each other. They can kind of act as an electron pair or make bromines 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 what we're going to see is that this is going to catalyze the reaction. So the iron bromide. We have some iron and it's bonded to three bromines just like that. So we haven't seen iron bromide much. But the way I think about it is, bromine molecules are much more electronegative than the iron. So even though these look like fair covalent bonds, if we had 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 is that this iron will have a slightly positive charge. Because the electrons are being hogged away 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 a Lewis acid will accept an electron. So this might want to act as a Lewis acid. So who can it nab electrons from? Well maybe it can nab an electron from this bromine right here. And I'm not saying that this 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 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 and it's attached, of course, to the three bromines. 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. And 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. And the iron, now that it gained this electron, now has a negative charge. So let's think about what's going to happen now. Now we're going to bring the benzene into the mix. So let me redraw the benzene. 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 the 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. Bromines are really, really, really, electronegative. You might see them with a negative charge, but the positive charge, it really wants to grab an electron. And in the right circumstances, you can imagine where it really wants to grab that electron right there. So maybe, if there was 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 than this bromine would have a positive charge, which isn't 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 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-- let me draw some hydrogens here just to make things clear. We already have hydrogens on all of these carbons. Sometimes it's 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 goes to the bromine. Maybe I should even do it this way. Just to make it clear. Kind of replaces that one. Although, 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 the electron right here goes to the blue bromine. 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. So let me draw the benzene ring. This double bond, that double bond. Let me draw all of the hydrogens. I want to do them in purple. So we have one hydrogen, two hydrogens, three hydrogens, four hydrogens, five hydrogens, and six hydrogens. 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. I've kind of flipped it around and now it has its other six valence electrons. One, 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 aromatic benzene ring like this. But it's not a ridiculous carbocation. 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? Well this bromine had a positive charge. Now it gained an electron. Let me draw it. So you have your bromine. It gained an electron. So now it is neutral again. So let's see, it had the one, two, three, four, five, six. Now it gained this blue electron. So now it's seven valence electrons. Back to being neutral. It's bonded to the iron bromide. Let me draw the bromines. One, two, three. And so we've given this blue bromine to the benzene ring. But it's not happy here. It doesn't want to break it's aromaticity. 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. So you can imagine-- and 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 rid of one of bromines, 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 nabs the proton, than the leftover electron is still there. That electron is still there. Let me do that in a different color. I'll do it in red. That electron is still there. And then that electron can be given to that original carbocation and we'll have a nice aromatic ring again. So how would that look? So you could imagine a situation where this electron, this green electron right here, gets 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 carbocation And then what are we left with? Well, we have our-- let me 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 hydrogens. 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 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 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. So we have our iron bonded to three bromines. We are bonded to three bromines just like that. It has lost its negative charge. And this electron right here has now gone to the carbocation So that electron right there has now gone to the carbocation. It is right there. So this bond, 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 we now have this bromo group there. So we have this orange electron is right over here. And 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 canceled 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 on what happened here. 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, it became this larger molecule. Now this whole thing can act as a pretty strong electrophile. This grabbed an electron. It broke the aromaticity of the benzene ring. But just long enough for this bromine to form. But once this bromine, it gained an electron and then bonded to the benzene. 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, nabs the proton, allows this electron to go back to that carbocation and then we're left with the iron 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.