In this video we're going to take a look at the halogenation of benzene. Here, benzene would react with either Br2 or Cl2. In this reaction, one of the hydrogen atoms on the benzene is substituted with an X, which stands in for either bromine or chlorine.
Here we'd say that a catalyst in a form of FeX3 where X3 is either Br3 or Cl3 must be used and has to contain the same halogen as the reagent. So basically what I mean by this is if X2 here is Cl2 then this has to be FeCl3. If X2 here is Br2 then this has to be FeBr3.
So if we take a look here at this benzene halogenation reaction example, we have our benzene with its six hydrogens on it. Remember, X here equals either bromine or chlorine. What happens is that one of these hydrogens gets replaced.
Hydrogens gets replaced by this halogen. Here, one of them. So here the H is now gone, and instead in its place we put one of the halogens. That halogen could be chlorine or bromine. This is a way that we can substitute in a halogen at the expense of losing a hydrogen on benzene.