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Topic: Predict reagent for given substrate  (Read 7742 times)

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sweetdaisy186

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Predict reagent for given substrate
« on: November 05, 2005, 11:11:46 PM »
Hey guys!

I have attached the problem that I have. I am supposed to prodict the reagent that would be required to carry out the given transformations. I am not asking for an answer, but a push in the right direction. Where should I start? What should I be looking for when I am working these types of problems? Is there a checking system so that I know my answer is correct?

Thanks for any type of help. I greatly appreciate it!

Offline Jiro

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Re:Predict reagent for given substrate
« Reply #1 on: November 06, 2005, 01:02:08 AM »
haha nice i think this is question 40 in chapter 4 in the text book "Organic Chemistry" by Paula Yurkanis Bruice... read the chapter on Reaction of Alkenes and that should put you in the right direction...remember... electrophile...nucleophiles... etc.

sweetdaisy186

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Re:Predict reagent for given substrate
« Reply #2 on: November 06, 2005, 01:14:46 AM »
This is a question from a textbook? We use a different textbook. My problem is that I can't tell the difference between what is supposed to be the nucleophile and what is supposed to be the electrophile. The nucleophile has a negative charge, correct?

Offline Jiro

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Re:Predict reagent for given substrate
« Reply #3 on: November 06, 2005, 01:32:37 AM »
If I take HCl for example and put it into water... it will contain to ions H+ and Cl-... the hydronium ion is an electrophile because it wants electrons... the Cl ion is a electron rich atom hence nucleophile because it wants to attach to carbocations in this case...
Also it is good to note that the electrophile tends to be attracted towards hydrogen rich areas which would result in a more stable carbocation...(Markovnikov's rule)... Also Br-Br  verses H-H with Pd/C should be noted of there differences (syn and anti)... Anyway there are alot of different types of reactions you should familiarize yourself with! So read! Practice!

You should find the mechanisms between those products... I highly recommend reading a chapter in your text book that goes along the lines of "reactions of alkenes" you will probably progress faster doing so.

sweetdaisy186

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Re:Predict reagent for given substrate
« Reply #4 on: November 07, 2005, 04:27:20 PM »
LOL, thanks for the advice! I will let you know how it goes!  ;D

sweetdaisy186

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Re:Predict reagent for given substrate
« Reply #5 on: November 07, 2005, 05:17:13 PM »
I have read over the sections in my book and came to following conclusions, that I hope are correct...

Starting with the compound with the two bromines and going clockwise...

If we are using the compound in the middle to make the compound at the top, I think the double bond would attack Br2 (my theory for the reagent). This would split the Br2 into two radicals. Since the double bond has broken, there are two radicals on the compound as well; allowing room for the bromines to attach onto the compound.

The next compound with no substituents hanging off, I believe the reagent to be H2 for the same reason as above.

The next two compounds I believe use HBr because I believe that the double bond breaks the HBr, and one H goes to one radical that came from the broken bond while the Br goes to the other radical on the compound.

The compound with the OH and Br, I believe the first reagent is HBr and then OH because the double bond will break the HBr, allowing the Br to attach to the compound, and then the OH instead of the H will attach to the other part of the compound where the other radical is located.

The compound with the OCH3, I believe the reagent is OCH3

The last two compounds with the OH only, I believe has a reagent of OH for the same reason as the two compounds with the Br.

Any imput on my theories would be greatly appreciated!

sweetdaisy186

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Re:Predict reagent for given substrate
« Reply #6 on: November 07, 2005, 05:54:24 PM »
I am going to change my answer for the last two compounds to the hydronium ion....

Offline Jiro

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Re:Predict reagent for given substrate
« Reply #7 on: November 07, 2005, 11:10:30 PM »
seems like you have made some progress haha

Quote
If we are using the compound in the middle to make the compound at the top, I think the double bond would attack Br2 (my theory for the reagent). This would split the Br2 into two radicals. Since the double bond has broken, there are two radicals on the compound as well; allowing room for the bromines to attach onto the compound.

The next compound with no substituents hanging off, I believe the reagent to be H2 for the same reason as above.

basically true but there is a difference you should know between halogenation and hydrogenation in this case Br2(halogen) which isnt really shown very well in this example... there are times when the attack of the nucleophile can be stereoselective!

Quote
The next two compounds I believe use HBr because I believe that the double bond breaks the HBr, and one H goes to one radical that came from the broken bond while the Br goes to the other radical on the compound.

Ya but why is it attached differently? and does HBr really break the double bond?
H+ from HBr molecule is the one that sort of breaks the double bond intially... HBr is a good answer for the one with the Br at the secondary carbon but the next one is then another question.

You should break it down with figuring out the electrophile and nucleophile first then determine where the electrophile will go thus making a carbocation(regioselectivity) then determine whether or not you have to think about the stereoselectivity of the nucleophile(ANTI or SYN)... venturing further, if you have more than one reactants then you will ultimately have more nucleophiles to choose from. This will definetely alter your final product if you choose the wrong nucleophile to attack the carbocation. so therefore, when this happens, you must ask yourself which is the better nucleophile?
« Last Edit: November 07, 2005, 11:13:23 PM by Jiro »

sweetdaisy186

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Re:Predict reagent for given substrate
« Reply #8 on: November 08, 2005, 12:15:36 AM »
Hmmm, the double bond will break the HBr, not the other way around. Whoops, I knew that, I promise!  :-[
The hydrogen would be a better electrophile and the Br would be a better nucleophile, in my opinion....I will have to think about this one. I will be back.... LOL Thanks for your *delete me* You are making me think of things that I never thought of before!

Arsenic

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Re:Predict reagent for given substrate
« Reply #9 on: November 08, 2005, 05:32:28 PM »

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