Why Berkley Review Why??? =(

[johnwandering]

I've gotten a question wrong on one of my practice tests because I distinctly read in the BR that electrophilic-addition reactions require a Polar/Protic solvent~~


I came on to a reagent and solvent choice and the right answer for the electrophilic-addition reaction was
Br2/CCl4
Which is neither polar nor protic


What is up with this?
Did BR just drop the ball here??

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[Don Draper]

I've gotten a question wrong on one of my practice tests because I distinctly read in the BR that electrophilic-addition reactions require a Polar/Protic solvent~~


I came on to a reagent and solvent choice and the right answer for the electrophilic-addition reaction was
Br2/CCl4
Which is neither polar nor protic


What is up with this?
Did BR just drop the ball here??

Please list all 4 answer choices.

Remember the MCAT is a test of finding the correct multiple choice answer, it is not a test of having the perfect answer. Likely none of the other 3 choices you had worked.

If you go into this test trying to find the textbook answer everytime, you will second guess yourself and be very slow.

If...

A. Horrible
B. Sucks
C. Abysmal
D. Pretty correct but not textbook correct

Then choose D, get the answer right and move to the next question.

 

[johnwandering]

Sorry, I do not remember the question (it was noted in my notebook of corrections with exclamation marks and stars all over it)


The thing is, how is this remotely correct?
I was under the assumption that elimination would occur and reverse the reaction if the solvent was aprotic, much less nonpolar~

 

[johnwandering]

I can give you a rough memory of the problem, thought I can't write the exact one



A picture of an alkene is shown with a reaction arrow with the Br2/CCl4 reactants on it (A regular alkene hydrocarbon with no steric hindrances)
the question then asked us what would occur with a series of molecules to choose from
I can only remember the two relevant ones:
One with the electrophilic-addition product of attatched Br
and One with the same reactant alkene (essentially no reaction)


I don't understand why no reaction was not the answer as the Berkley Review seems to clearly state that a polar and protic solvent is needed or else elimination might occur and the rxn may reverse~~~



please help!!!
=(

 

[johnwandering]

hmmmm
I found this online

http://www.youtube.com/watch?v=YnD42ZTPdFU


which seems to indicate CCl4 is a suitable solvent for this reaction.
So I am guessing that polar/protic solvent is not needed and I can ignore the BR??

 

[Rayhaan]

The alkene is acting as an electron rich site which will react to add bromine.

 

[johnwandering]

Yes, I know this


I am wondering why CCl4 is a suitable solvent for electrophilic addition, when the BR clearly indicates that elimination will occur to reverse the reaction unless the solvent is polar and protic

 

[Don Draper]

Yes, I know this


I am wondering why CCl4 is a suitable solvent for electrophilic addition, when the BR clearly indicates that elimination will occur to reverse the reaction unless the solvent is polar and protic

John,

unfortunately this is a multiple choice exam and not a fill in the blank exam. The validity of an answer is not only based on the answer BUT also the other 3 answers. Find the multiple choice answers and post it.

You can post a question, it's done all the time. You post the stem and the 4 answer choices. Don't be so scared (or are you not wanting to take the time to find it?).

 

[johnwandering]

Hi Don~~
Thanks for the help, I really do appreciate it (this this has been worrying me because it indirectly affects the validity or rather mental rigidity of all of the solvent choices I've memorized from the berkley review)
I'll make another effort to describe the issue


I've been looking for the exact question, but I cannot find it
It is somewhere within my Mcgraw-Hill practice tests and my Princeton Review science workbook from 2008~
But I can remember the question:

It is NOT a passage question
A picture of a Trans-2-Butene is shown with a reaction arrow with the Br2/CCl4 reactants on it
the question then asked us what would occur, allowing us to choose from a series of molecules
a.) A butene with trans Bromination on either side of the alkene
b.) A the regular butene (no reaction has occurred)
C.) a butane
D.) a butane with two bromines on the 2nd carbon

the answer is A!
The inquiry here was:

"Why is the answer A and not B??"



The Berkley Review clearly states in its content review that the electrophilic addition reaction requires a POLAR and PROTIC solvent, or else elimination would occur and NO REACTION will happen~~
In this question, a nonpolar aprotic solvent was used--------- why did reaction occur?


But to raise the specificity of the question further, I found this video on youtube which clearly states that electrophilic addition reaction CAN in fact occur in a CCl4

http://www.youtube.com/watch?v=YnD42ZTPdFU

This means that the question was infact correct, and that electrophilic addition reaction DOES occur in the presence of a nonpolar/aprotic solvent




I am just wondering why BR put this bit of misleading information into the book that would have caused others like me who encountered this question to get it wrong.
Also, this worries me because now everything I've really known about solvents I have to forget, lest I encounter more troubling problems like this.


Or perhaps I am misunderstanding something here, and you can inform me?

 

[Don Draper]

Hi Don~~
Thanks for the help, I really do appreciate it (this this has been worrying me because it indirectly affects the validity or rather mental rigidity of all of the solvent choices I've memorized from the berkley review)
I'll make another effort to describe the issue


I've been looking for the exact question, but I cannot find it
It is somewhere within my Mcgraw-Hill practice tests and my Princeton Review science workbook from 2008~
But I can remember the question:

It is NOT a passage question
A picture of a Trans-2-Butene is shown with a reaction arrow with the Br2/CCl4 reactants on it
the question then asked us what would occur, allowing us to choose from a series of molecules
a.) A butene with trans Bromination on either side of the alkene
b.) A the regular butene (no reaction has occurred)
C.) a butane
D.) a butane with two bromines on the 2nd carbon

the answer is A!
The inquiry here was:

"Why is the answer A and not B??"



The Berkley Review clearly states in its content review that the electrophilic addition reaction requires a POLAR and PROTIC solvent, or else elimination would occur and NO REACTION will happen~~
In this question, a nonpolar aprotic solvent was used--------- why did reaction occur?


But to raise the specificity of the question further, I found this video on youtube which clearly states that electrophilic addition reaction CAN in fact occur in a CCl4

http://www.youtube.com/watch?v=YnD42ZTPdFU

This means that the question was infact correct, and that electrophilic addition reaction DOES occur in the presence of a nonpolar/aprotic solvent




I am just wondering why BR put this bit of misleading information into the book that would have caused others like me who encountered this question to get it wrong.
Also, this worries me because now everything I've really known about solvents I have to forget, lest I encounter more troubling problems like this.


Or perhaps I am misunderstanding something here, and you can inform me?

John,

I believe this will come down to testing skill. If you have a butene and they are adding Br2 then the test writers are likely testing if you know this reaction. They likely are not trying to test your knowledge of solvent effects. I would know general solvent effects for nucleophillic/electrophillic rxns (SN1/SN2), but as for them testing if you know that no rxn will happen in certain solvents is unlikely.

Test writer: Lets see if they know that by using the wrong solvent, this rxn won't happen.

OR

Test writer: Lets see if they know this rxn.

Obviously if the passage talks about solvents, then this may change. But for you to know electrophillic add'n solvent effects is likely not tested.

By the way, did you know the word solvent only appears twice on the bio topics list for the MCAT? Once for extraction techniques and once for recrystallization.

Always return to the goal. To get answers correct on the MCAT.

OCHEM TOPICS...
http://www.aamc.org/students/mcat/preparing/bstopics.pdf

 

[Rayhaan]

You don't need a protic solvent to prevent it reversing, you need a weak base as a nucleophile which is what happens here. Br-Br, one takes the electrons so you get a Br+ and a Br-. The Br+ electrophilically adds. That creates a carbocation. The Br- can add. This is your weak base that would rather attach to the carbocation rather than cause an elimination. I think you just over memorized here. Really though this is one of those reagents/solvents where if you've done it enough times you should be familiar with it (it was for me since I've seen it so many times in orgo 1 and 2).

 

[BerkReviewTeach]

Hi Don~~
Thanks for the help, I really do appreciate it (this this has been worrying me because it indirectly affects the validity or rather mental rigidity of all of the solvent choices I've memorized from the berkley review)
I'll make another effort to describe the issue


I've been looking for the exact question, but I cannot find it
It is somewhere within my Mcgraw-Hill practice tests and my Princeton Review science workbook from 2008~
But I can remember the question:

It is NOT a passage question
A picture of a Trans-2-Butene is shown with a reaction arrow with the Br2/CCl4 reactants on it
the question then asked us what would occur, allowing us to choose from a series of molecules
a.) A butene with trans Bromination on either side of the alkene
b.) A the regular butene (no reaction has occurred)
C.) a butane
D.) a butane with two bromines on the 2nd carbon

the answer is A!
The inquiry here was:

"Why is the answer A and not B??"



The Berkley Review clearly states in its content review that the electrophilic addition reaction requires a POLAR and PROTIC solvent, or else elimination would occur and NO REACTION will happen~~
In this question, a nonpolar aprotic solvent was used--------- why did reaction occur?


But to raise the specificity of the question further, I found this video on youtube which clearly states that electrophilic addition reaction CAN in fact occur in a CCl4

http://www.youtube.com/watch?v=YnD42ZTPdFU

This means that the question was infact correct, and that electrophilic addition reaction DOES occur in the presence of a nonpolar/aprotic solvent




I am just wondering why BR put this bit of misleading information into the book that would have caused others like me who encountered this question to get it wrong.
Also, this worries me because now everything I've really known about solvents I have to forget, lest I encounter more troubling problems like this.


Or perhaps I am misunderstanding something here, and you can inform me?

I'm hoping my post can help calm your nerves a bit. I worry about you a little, because you can feel the anxiety in many of your posts here in the Q and A forum, and that can be counterproductive. The right amount of stress can be great for motivation and focus, but if you're too high strung, it hurts your performance. Just roll with the flow and pick up as much as you can, and try not to get hung up on the little minutiae. This is a perfect example.

First and foremost, Br2 addition to an alkene is NOT tested on the MCAT. I have been through the BR book chapter 4 three times now looking for the Br2 reaction you are talking about, and it is not in there. The book only addresses reactions that are tested.

The book shows a generic = + E+ to form a carbocation reaction. In Figure 4-15 it says "Solv: represents any polar/protic solvent." Nowhere does it say that electrophilic addition can only be done in a polar/protic, but it does say that the mechanism shown is best done under acidic conditions and that you need to think about rearrangement. Be careful not to read too much into this. The most important sentence in the entire section on addition reactions is the first one, stating that alkene reactions are not tested, but dienes are. You can add H+ and a nucleophile such as H2O or X- across a diene by a mechanism similar to the one shown in Figures 4-14 through 4-16. That is the takehome message!

The MacGraw Hill question is not an MCAT question. It is definitely true that Br2 reacts in the aprotic/nonpolar CCl4, which is what the old BR orgo books showed before the MCAT test writers removed the topic in 2003.

I'd like to address your comment about "or else elimination would occur and NO REACTION will happen~~", but I can't find that anywhere in the version of the organic chemistry book I have. Where does it say that addition runs the risk of elimination? Those two reactions are in essense reverse reactions of one another. Addition reactions require some form of a nucleophile to attack the carbocation, and often that nucleophile is the polar/protic solvent, otherwise it will remain as the alkene reactant. Is that what you mean?

 

[johnwandering]

Thanks an absolute million!!


I guess I am so used to studying for those premed courses, that it haunts me when there are gaps in my knowledge


I will approach the exam with a better mind set
=)

 

[bullsfan1986]

Not to hijack this thread or anything, but if you had Cl2 instead of Br2 would you get a "cis" chlorination due to the fact that the Cl2 addition would be faster and thus less selective of product stability?

 

[Rabolisk]

I don't think so. Last I checked the reaction is trans because of the bridged intermediate. Not sure that chrolination would change that. anyhow, it's not something you need to know for the mcat.

 

[Don Draper]

MCAT - applying basic science concepts in difficult situations

Not MCAT - knowing what happens in special circumstances