 In this video, we are going to discuss how to prepare hallow arenas by electrophilic aromatic substitution reaction. So as the name suggests, we are substituting a hydrogen atom with a halogen atom to give hallow arene. But the interesting thing is that the halogen like chlorine or bromine is not sufficiently electrophilic to react with benzene. This is because from this halogen alone, generation of an electrophile like Cl+, or Br+, is actually difficult. And that is why we need the assistance of a catalyst like a Lewis acid like FeCl3 or FeBr3 so that it can form a complex with a halogen atom to form a much stronger electrophile. Benzene can react with this stronger electrophile and produce the desired hallow arene. So let's now look at the mechanism of this reaction. As we mentioned before, for the electrophilic aromatic substitution reaction to take place, we need a very strong electrophile and that can be achieved with the help of the Lewis acid. For example, here we have bromine atom and in the presence of a Lewis acid like FeBr3, it can form a complex like this which is Br2, FeBr3. Now what makes this a strong electrophile is the weakened bromine-bromine bond and a partial positive charge on one of the bromine atoms. We know that bromine is an electronegative atom and it does not do well when it has a positive charge attached to it. It has to lose its positive charge and acquire electrons which makes it behave like a very strong electrophile. Now in the next step where the electrophilic attack by benzene takes place, the pi electrons of the benzene attacks this bromine and forms sigma complex as you can see here. Now remember guys, this formation of the sigma complex is a rate limiting step. This step is strongly endothermic because it forms a non-aromatic carbocation. In the last step, a proton is lost and aromaticity is regained. Here FeBr4- has a negative charge and the bromine ion here acts as a weak base and abstracts this hydrogen atom from the sigma complex. And this removal of hydrogen atom restores the aromaticity and gives us the final haloarine. And in this process an acid HBr is produced and our catalyst FeBr3 is regenerated. Now because in this step aromaticity is regained, this reaction or this step is actually exothermic. And the overall reaction is exothermic by 45 kJ per mole. Now chlorination of benzene also follows the same mechanism and works in the presence of Lewis acids like FeCl3 or AlCl3. Now iodination of benzene or substituted benzene requires the presence of an acidic oxidizing agent like nitric acid. This is because reactions with iodine are usually reversible and we need to oxidize iodine to generate the electrophile which is I+. Now here nitric acid is not a catalyst but a reagent because it gets consumed in the reaction. Lastly, we cannot use this method to produce fluorocompounds because of the very high reactivity of fluorine. Now this high reactivity of fluorine makes it very difficult to control the reaction. So this is how haloarines can be prepared via electrophilic substitution reactions.