Chemistry 3B Lecture 8 - Chemistry 3B Lecture 8 Tuesday...

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Chemistry 3B Lecture 8 Tuesday February 17 th , 2009 Electrophilic Aromatic Substitution: for bromination, we need a catalyst. The mechanism is shown, and you need to know how the Br+ came from the FeBr3. The electrophile adds, generates a resonance stabilized carbocation which is no longer aromatic. We take a hydrogen off of the carbon that bears the electrophile and we re-aromatize the ring. Nitration: We use the nitro group when it is attached to a benzene ring a lot in organic chemistry because it is a versatile group. This is clearly an EAS reaction in the sense that forget about the products; just look at the fact that the H was replaced with a nitro group. The only thing with EAS is how you generate the electrophile. Write down the structure of HNO3 (nitric acid). Sulfuric acid is not consumed. It is tempting to protonate the OH- with the sulfuric acid, but this won’t help because you need a water, and you’d have to protonate it again. It doesn’t help. We can protonate the OH and the water will fall off. There are 2 plus charges next to each other so the water will come off easily. This is called the nitronium ion. The counter ion is bisulfate, which we got originally by protonating the nitric acid. You can lose the water first, and then you will get a 2+ charge on the Nitrogen. After that the oxygen can stabilize it. The nitronium ion will be a really powerful electrophile. We have generated the water so we don’t have to worry about that anymore. Now we can generate the nitrobenzene. Bisulfate will be pulling off the Hydrogen.
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Chemistry 3B Lecture 8 - Chemistry 3B Lecture 8 Tuesday...

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