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Unformatted text preview: Electrophillic aromatic substitution. When you have an aromatic compound like benzene, you can do a substation reaction. Remember, in nucleophillic substitution, you have an electrophile and a nucleophile attacking, like bromine attaching across a double bond. The double bond is the nucleophile. This is what we see, the double bond attacks the electrophile and we get the carbocation intermediate. Now, in the case of the addition across the double bond is the nucleophile attacking the intermediate and you get substitution, but aromatic systems don’t undergo this next step. Instead,the nucleophile acts as a base and removes the proton. You substitute the hydrogen on the aromatic system for the electrophile. We don’t see the nucleophile attacking the aromatic product and ending up with the addition product because in order for the nucleophile to act as one, it has to overcome a huge activation energy in order to attach because aromaticity being preserved is preferred, that’s why we see the reformation of the benzene ring after the reaction has taken place. What happens if there is another functional group on the ring? As soon as we attach it, we notice that it does matter which one to remove because there are three possible products from the reaction, ortho, next to functional group, meta, two away, and para, the carbon furthest away. There are three possible products we can get, but now we have to know which one we are going to get. There is a control over the substitution, it depends on what functional group you have. There are two types of group, electron withdrawing and electron donating. To figure out which one affects which carbonwise, we need to take a look at the carbocation intermediate. We have an aromatic compound, and we undergo at the carbocation intermediate....
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