E1 vs E2

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chiddler

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2-bromopropane and sodium ethoxide in ethanol. The major product is an elimination product.

E1 or E2?

TBR says that generally speaking, it can swing either way when its a secondary carbon. And I remember from ochem class that generally SN2 or E2 are more common on secondary carbons. Which turned out to be correct for the passage question I was answering.

But i'd like a more solid understanding. I know the effects of temperature, how bulky the base is, quality of LG, etc.

My question: Is there any distinguishing aspect of this reaction that should tell me it is E2 besides knowing secondary carbons usually do X2's?

edit: doing some reading. i think the answer is probably not much else to it.
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Probably, the fact that NaEtO is a STRONG base, so therefore E2, otherwise, you need a good Lewis base for E1?
Of course, Tertiary are favored for E1 because of stabilization of the carbocation after LG has left the building.
Also polar protic solvents can be used to hinder nucleophiles, preventing E2 and SN2
 
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chiddler said:
2-bromopropane and sodium ethoxide in ethanol. The major product is an elimination product.

E1 or E2?

TBR says that generally speaking, it can swing either way when its a secondary carbon. And I remember from ochem class that generally SN2 or E2 are more common on secondary carbons. Which turned out to be correct for the passage question I was answering.

But i'd like a more solid understanding. I know the effects of temperature, how bulky the base is, quality of LG, etc.


My question: Is there any distinguishing aspect of this reaction that should tell me it is E2 besides knowing secondary carbons usually do X2's?

edit: doing some reading. i think the answer is probably not much else to it.
Click to expand...

Hmm...that's a good question. Maybe because sodium ethoxide is a strong base? E2 is favored by a strong base. Weak base is SN2. I think. I may be totally backward on this, but I figure that I'll learn something by being brave and posting. :)
Yikes...I need to work on orgo.
 
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I was just about to post a Kaplan problem exactly on E1 vs. E2!

Kaplan gave me just this answer: It's E2 because ethoxide is a strong base (which I didn't know) and E2 favors strong bases, while E1 favors weak bases.
 
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The thing is with this E2 reactions, I rarely see "common bases". So methoxide (CH3O- in CH3OH) and ethoxide are strong bases.

Kaplan says "strong bulky bases = E2". Anyone know any common MCAT ones?
 
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strong bases: OH-, RO-, R2N-
All of these would love to become H2O Pka 14, ROH pka 17?, R2NH pka 35-36
 
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SaintJude said:
The thing is with this E2 reactions, I rarely see "common bases". So methoxide (CH3O- in CH3OH) and ethoxide are strong bases.

Kaplan says "strong bulky bases = E2". Anyone know any common MCAT ones?
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Methoxide would probably favor SN2 over E2, but if you are told its an elimination, strong base will always indicate E2 over E1. A bulky base means E2 over SN2, but a strong and bulky base is always going to be E2.

Tert-butoxide is a common E2 base.
 
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Tert butyl oxide and LDA are som very common bulky bases that favor E2 no matter what.
 
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I agree with the fact that it is a strong base favoring E2, but what about the solvent? (I always get confused on how much of a "voice" the solvent has).
Ethanol is a protic solvent, right? Shouldn't that component of the reaction favor E1? Or is there a hierarchy of factors to consider?
Thanks!
 
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Oh alkenes, how I"m glad I never learned those :rolleyes: looked at? sure. learned? no. Not efficient use of time, unless other stuff is 100% or preparing for something other than the MCAT.
 
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RetailBoy said:
Oh alkenes, how I"m glad I never learned those :rolleyes: looked at? sure. learned? no. Not efficient use of time, unless other stuff is 100% or preparing for something other than the MCAT.
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Except E1/E2 are not alkene reactions.
 
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MedPR said:
Except E1/E2 are not alkene reactions.
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I agree, partially. E2 always ends up with an alkene right? I'm not sure, I'm definitely not as well educated on E1/E2 stuff as you so I could be totally wrong.

The way ochem MCAT passages (most info needed is in passage, might need basic ochem rules + attracks -... wait that's gchem :p ) are in addition to these reaction very frequently involving alkenes especially E2 (I believe it always does always)
 
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MedPR said:
Except E1/E2 are not alkene reactions.
Click to expand...

I laughed at the way you solved that spring box question BTW in the other thread. Because when I read the question too before reading your post I solved it the exact way --- TBR has taught us well. :love:
 
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gettheleadout said:
What do you mean partially? Yeah elimination reactions result in double bonds, but "alkene reactions" are things like additions.
Click to expand...

Topic list specifically mentions SN1 SN2. Doesn't mention Alkenes/E1/E2/elimination.

Not like it matters what an 'alkene' rx is: but alkenes are on the same level of alkanes (if oyu know what I mean). So If they have alkanes on the list (which the do) and they wanted to have alkenes on there too they would have treated it the same (which they did several years ago, they specifically put alkenes on there, one day- bam gone). As a result I wouldn't expect there to be any information we need to know on how things interact with alkene's specifically (that doesn't apply to anything else, ie answer will be able to be determined without /any/ knowing info about alkenes, except maybe they ar ein a molecule and not in spaghetti).
 
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But seriously, I am horrible with the E1E2 alkene topic (for a good reason) SO I could easily be wrong. But I do know that topic list pretty damn well.
 
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I thought elimination reactions were on there somewhere? Idk though, maybe I studied it for no reason!
 
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it's not there. i remember looking a while ago.

every single prep company has it on their list. so i assume that aamc just...lies...or something. i don't know.

why would they all teach it?!
 
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chiddler said:
it's not there. i remember looking a while ago.

every single prep company has it on their list. so i assume that aamc just...lies...or something. i don't know.

why would they all teach it?!
Click to expand...

1 out of date.
2 it could very well be in a passage. Like most PCR stuff (if not all) is in passage, but still taught.

Just what I think
 
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Does TBR still have alkene stuff in their most recent books? BerkReviewTeach has commented before on the lack of need to study the removed material (alkenes, benzenes, alkynes, etc...) so I'd be surprised if the books still include it.
 
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chiddler said:
it's not there. i remember looking a while ago.

every single prep company has it on their list. so i assume that aamc just...lies...or something. i don't know.

why would they all teach it?!
Click to expand...

The same reason why they teach rotational motion; for the sake of completeness.
 
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gettheleadout said:
Does TBR still have alkene stuff in their most recent books? BerkReviewTeach has commented before on the lack of need to study the removed material (alkenes, benzenes, alkynes, etc...) so I'd be surprised if the books still include it.
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He's mentioned several times over past years that alkenes are not on the test - people are surprised every time (understandably though: its in prep books and the OLD 1-2 mcats, and I saw at least 3 passages on E1E2 and benzenes on GS's first five CBT test). But I'm sure they still do - with how thorough TBR is - and I would want my test book to too (why I used TBR) so reading the passage/mechanism would be a so easy.

BUT I agree I wonder if the have it a lot more deemphasized (I believe they have 2 passages on it, it it slips in on a couple random Q's = old book - actually not sure if the updated the ochem book - I know phys they did)
 
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Wait, so if E1/E2 reactions are not on the exam, does that mean we assume all nucleophillic attacks result SN1/SN2?
 
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hellocubed said:
Wait, so if E1/E2 reactions are not on the exam, does that mean we assume all nucleophillic attacks result SN1/SN2?
Click to expand...

I am 100% positive I've seen a question on AAMCs that specifically asks for the elimination product. Though it could've been explained in the passage.
 
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MedPR said:
I am 100% positive I've seen a question on AAMCs that specifically asks for the elimination product. Though it could've been explained in the passage.
Click to expand...

They were on there. But elimination used to be on MCAT. 10 and 11 didn't.
 
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chiddler said:
protic favors both SN1 and E1.

E1 specifically is favored with bulky base and high temps.
Click to expand...

sorry to keep coming back to this, but then the solvent (ethanol) is protic, thus it should favor E1 over E2. right? :confused:
 
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pm1 said:
sorry to keep coming back to this, but then the solvent (ethanol) is protic, thus it should favor E1 over E2. right? :confused:
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Still going to be E2 because ethoxide is a strong base.
 
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Given a good leaving group, E1 favored.

don't forget leaving group quality. hasn't been mentioned yet.
 
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MedPR said:
Still going to be E2 because ethoxide is a strong base.
Click to expand...

okay! So there is some kind of hierarchy of factors to look at then? Like in this case the strong base speaks louder than the solvent?
 
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wait ethanol solvent says nothing about quality of base that is also added. what if a weak base like NaCN? that won't deprotonate the ethanol and it may perform E1.
 
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pm1 said:
okay! So there is some kind of hierarchy of factors to look at then? Like in this case the strong base speaks louder than the solvent?
Click to expand...

chiddler said:
is that right?

what is an example of a protic medium that allows E1 then?
Click to expand...

chiddler said:
wait ethanol solvent says nothing about quality of base that is also added. what if a weak base like NaCN? that won't deprotonate the ethanol and it may perform E1.
Click to expand...

I learned that a strong base almost always means E2. Basically a strong enough base can initiate the E2 before the leaving group falls off and initiates E1. Any weak base will promote E1 over E2.
 
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