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Topic: E1 or E2?  (Read 413 times)

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Offline Emmyg1979

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E1 or E2?
« on: February 07, 2020, 04:36:51 PM »
I am debating a simple organic chemistry reaction and hoping you guys would take the time to settle this debate...

1-bromo-1-methylcyclohaxane is reacted with tBuOK in tBuOH.

Is the reaction E1 or E2 & why?


No other information is provided for this question and the crux is two methods for teaching reaction mechanisms are providing two answers.

Thanks!

Offline AWK

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Re: E1 or E2?
« Reply #1 on: February 07, 2020, 04:46:49 PM »
Read your textbook or al least Wikipedia.
No other information is needed.
AWK

Offline Emmyg1979

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Re: E1 or E2?
« Reply #2 on: February 07, 2020, 05:09:20 PM »
Certainly that sounds obvious, I am asking because there has been a discrepancy. 

My professors teaching method emphasizes the possible formation of a carbocation; explaining that this is an E1 reaction because the bulky base takes so long to get to the molecule which is stericly hindered (ie the reaction rate is significantly slower), and in the mean time the carbocation is formed due to a good leaving group and protic solvent which increases the rate of carbocation formation.

All other resources state simply that a strong bulky base with a tertiary alkyl halide will proceed through an E2 mechanism because the rate determining step of this reaction is much faster than that of an E1mechanism.

My research (I did read the textbook; I have done research, I know that in 1956 Brown, et al demonstrated that steric hindrance does not effect basicity, but I can't find anything more recent; I understand that tBuOH is a common solvent for tBuOK reactions; I know that tBuOH is a weakly polar solvent; I Understand the trends) and discussions have also supported the E2 reaction mechanism, with the single caveat that if the concentration of tBuOk was significantly low the reaction may proceed through an E1 mechanism.

I am asking because I respect my professor, and his credentials are sufficient enough for me to believe that he is not off the wall wrong.

Offline Emmyg1979

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Re: E1 or E2?
« Reply #3 on: February 07, 2020, 05:19:14 PM »
I would like to be really sure, and understand all of the variables so that I can either proceed with the understanding that this particular reaction (and others possibly like it) is a caveat to the bulky base rule, or be able to discuss this further with my professor with out sounding like a righteous jerk.

It would be great if anybody could give me supporting arguments for both sides, so far I only have support for one.

Offline sharbeldam

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Re: E1 or E2?
« Reply #4 on: February 07, 2020, 05:33:10 PM »
Usually a strong base prefers E2, if its bulky then you are right, it prefers to take the less sterically hindered hydrogen and the hoffman product would be major.
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Offline Emmyg1979

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Re: E1 or E2?
« Reply #5 on: February 07, 2020, 05:45:13 PM »
Thank you sharbeldam, I appreciate your response, and yes, that is my understanding, a strong, bulky base with any 1°, 2°, or 3° alkyl halide will proceed through an E2 elimination with Hoffman selectivity so long as an appropriate H is available. 

I have since found dozens of reactions with similar variables and they are all reported to go through E2; I cannot find this exact reaction, and would hate to approach my professor without significant evidence/support.  I came to the forum to seek either that or a better understanding of why/how this reaction might be different.

Online rolnor

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Re: E1 or E2?
« Reply #6 on: February 08, 2020, 04:27:08 AM »
What would you say about the startingmaterial, does it easily form carbocation?

Offline hollytara

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Re: E1 or E2?
« Reply #7 on: February 08, 2020, 11:28:32 AM »
The two variables not given are the temperature and concentration of base. 

At sufficiently high temperature, E1 could start to compete with E2; this will be amplified if the base concentration is relatively low so that E2 slows down. 

Typically E2 is the preferred route, so we usually would avoid these sorts of conditions.

Offline Emmyg1979

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Re: E1 or E2?
« Reply #8 on: February 08, 2020, 10:39:26 PM »
Thank you Hollytara & Rolnor,

Yes Rolnor, a carbocations could form given these conditions, but would it form before tBuO- attacks a beta H, given equal ratios of base to solvent at room temp? (Which is how I infer a lack of specific conditions).

It is my understanding that it would not unless, as Hollytara pointed out other specific conditions are met; ie low concentration of base or high temp to increase the rate of C+ formation.

Is this understanding  incorrect?

Offline hollytara

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Re: E1 or E2?
« Reply #9 on: February 09, 2020, 11:59:48 AM »
When the conditions are ambiguous, sometimes the only way to know the answer is to do the experiment.

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