polyhalogenation on benzene

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soby10

soby10

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I have a question regarding polyhalogenation. When benzene is treated w/ Ch3 or any alkyl group it should activate the ring right. B/C CH3- groups are ortho/para activators. So if the new activated ring is now treated w/ a halogen like Br souldn't it go through polyhalogenation at every o/p position on the ring? Or does it stop after only 1 Br in the ortho position as the minor product and 1 Br as para as the major product b/c halogens will decativate the ring? If this is the case when does polyhalogenation occur in general?
 
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If you JUST halogenate, it will stop after one or two depending on the time and conditions cause they are O/P deactivators....

however, if you have even ONE alkyl group anywhere, the other three positions will be halogenated immediately, the activating effects of it outweigh the deactivation of the one halogen.

So to answer your question, polyhalogenation will almost always occur when you have an activating group on the ring... To get around this, you can halogenate first, then Fri-Crafts afterward. so it goes O/P. ANother way is to deactivate a group. For example, if you added NH2... if you acetylate the amino group to make acetanilide, its now an o/p deactivator....
 
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soby10 said:
I have a question regarding polyhalogenation. When benzene is treated w/ Ch3 or any alkyl group it should activate the ring right. B/C CH3- groups are ortho/para activators. So if the new activated ring is now treated w/ a halogen like Br souldn't it go through polyhalogenation at every o/p position on the ring? Or does it stop after only 1 Br in the ortho position as the minor product and 1 Br as para as the major product b/c halogens will decativate the ring? If this is the case when does polyhalogenation occur in general?
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Why are you asking this? What MCAT review materials are you using? You need to throw them out immediately and get more updated ones. The MCAT people haven't tested benzene chemistry such as that in years. The good people at AAMC OFFICIALLY removed that topic in 2003.
 
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BerkReviewTeach said:
Why are you asking this? What MCAT review materials are you using? You need to throw them out immediately and get more updated ones. The MCAT people haven't tested benzene chemistry such as that in years. The good people at AAMC OFFICIALLY removed that topic in 2003.
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Well, thats good to know. Side note: I always wondered what i DO and DONT have to know from what I learned in class be it from physics, orgo, bio, etc... is there anyway for us to know what was officially removed and is officially still on the exam these days?
 
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Doodl3s said:
Well, thats good to know. Side note: I always wondered what i DO and DONT have to know from what I learned in class be it from physics, orgo, bio, etc... is there anyway for us to know what was officially removed and is officially still on the exam these days?
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At the AAMC website, there is a 25-page document that lists the subjects for all of the sciences. It's a little ambiguous in spots, but overall a really great place to start. What you'll find is that most MCAT materials have review sections on topics that aren't tested. You should cross-reference every section in the materials you are using with that list.
 
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Thanks for the info. i was going over my textbook i haven't started w/ a prep book yet but that's good to know i don't have to worry about benzene. How about other aromatics?
 
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