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Topic: Determine which compound undergos the faster E1 reaction  (Read 7107 times)

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Offline danteOne

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Determine which compound undergos the faster E1 reaction
« on: November 15, 2013, 07:25:06 PM »
I got just got test back and I got one question marked wrong that I think I got right. I would like some feedback to make sure that I am not completely wrong before going to talk to my professor. The question was "Rank these compounds in order of increasing rate of E1".

 I know that energy of activation determines the rate of the reaction and that intermediate carbocations stability is directly related to the energy of activation, so the question really was "rank these compounds in order of their carbocation stability".

The two compounds that she says I mixed up were 2-bromo-3-methylbutane and 3-bromo-1-pentene (shown below). She says that 3-bromo-1-pentene is more stable because "a carbocation with resonance is always more stable than a carbocation without resonance" (apparently that statement is always true even if the compounds are different) . I think that because both resonance structures for 3-bromo-1-pentene result in a carbocation at a primary carbon the resonance stabilization gained from the decolonization of the electrons will not be enough to compete with the stabilization caused by the carbocation being at a secondary carbon.
Below is a picture of the part of the structures from the part of the question I got wrong. 

Who do you think is right?

Offline Archer

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Re: Determine which compound undergos the faster E1 reaction
« Reply #1 on: November 15, 2013, 11:49:00 PM »


The two compounds that she says I mixed up were 2-bromo-3-methylbutane and 3-bromo-1-pentene (shown below). 


You have drawn 3-bromo-1-pentene incorrectly, you have drawn 3-bromopropene (allyl bromide) can this undergo elimination?

 3-bromo-1-pentene does not form two primary carbocations.
« Last Edit: November 16, 2013, 12:11:46 AM by Archer »
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Offline danteOne

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Re: Determine which compound undergos the faster E1 reaction
« Reply #2 on: November 16, 2013, 03:42:06 PM »
In the problem she gave us the structures, not the names of the compound. 3-bromopropene is what I meant to say. I tried to name the structures myself but I made a mistake.

My professor says that the compound, 3-bromopropene, undergoes an E1 reaction at a faster rate than 2-bromo-3-methylbutane because resonance stabilized compounds always undergo faster E1 reactions.

I know that Sn2 cannot occur at sp2 hybridized carbons. Is it also not possible to have E1 occur at sp2 hybridized carbons?


Offline orgopete

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Re: Determine which compound undergos the faster E1 reaction
« Reply #3 on: November 16, 2013, 05:11:40 PM »

My professor says that the compound, 3-bromopropene, undergoes an E1 reaction at a faster rate than 2-bromo-3-methylbutane because resonance stabilized compounds always undergo faster E1 reactions.


I'd argue with my professor on this one. Ask for the reference. If, however, the question included SN1 reactions, then I'd change my opinion.
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Offline zsinger

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Re: Determine which compound undergos the faster E1 reaction
« Reply #4 on: November 16, 2013, 08:34:08 PM »
I believe the rates would be VERY close.
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Offline Altered State

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Re: Determine which compound undergos the faster E1 reaction
« Reply #5 on: November 16, 2013, 08:47:57 PM »
I think this is some kind of missunderstanding...
In your professor's opinion, what would be the E1 product of 3-bromopropene? The hypothetical molecule would be an allene, and I really doubt that could be obtained by elimination of an alpha-haloalkene

Are you sure he wasn't talking about substitution reactions?

Offline danteOne

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Re: Determine which compound undergos the faster E1 reaction
« Reply #6 on: November 17, 2013, 12:00:50 AM »
I think this is some kind of missunderstanding...
In your professor's opinion, what would be the E1 product of 3-bromopropene? The hypothetical molecule would be an allene, and I really doubt that could be obtained by elimination of an alpha-haloalkene

Are you sure he wasn't talking about substitution reactions?

Yes, I am sure. The question asked which compound would undergo the faster E1 reaction. I think the test was written the night before, and there wasn't that much thought put into many of the questions. I will try to get some points back.
Thank you.

Offline Archer

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Re: Determine which compound undergos the faster E1 reaction
« Reply #7 on: November 17, 2013, 01:22:09 AM »
Ok, so you have already approached your Prof. and you were given the explanation

Quote
a carbocation with resonance is always more stable than a carbocation without resonance

So make sure you understand whether this statement about carbocations is correct regardless of whether or not it applies to the  question asked.

Ask about the relative rates of allene formation and where you can find this information (as Orgopete says, preferably the primary literature source)

Don't go in all guns blazing thinking that the academic has got it wrong, if they have made an oversight they may not appreciate being confronted on their competence at question writing.

Instead approach the Prof. with the attitude of general interest in allenes and the kinetics of their formation. If they have made a mistake they may thank you for highlighting it (they are only human after all).
« Last Edit: November 17, 2013, 04:27:27 AM by Archer »
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Offline bjams12

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Re: Determine which compound undergos the faster E1 reaction
« Reply #8 on: November 20, 2013, 06:16:19 PM »
Correct me if I'm wrong, but wouldn't your alkane carbocation undergo a hydride shift, thus making the compound a tertiary carbocation intermediate?

Offline spirochete

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Re: Determine which compound undergos the faster E1 reaction
« Reply #9 on: November 21, 2013, 12:51:47 AM »
Correct me if I'm wrong, but wouldn't your alkane carbocation undergo a hydride shift, thus making the compound a tertiary carbocation intermediate?

Yes but the rate limiting step would probably be formation of the secondary carbocation.

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