Mini-MCAT Passage: Alcohol Dehydration

[SaintJude]

Kaplan Section Test

Dehdyration reaction of secondary & tertiary alcohols is E1, and and E2 for primary alcohols.
How do you know which undergoes E1 & E2 ?

---

Members don't see this ad.

 

[pfaction]

The alcohol is most likely on Carbon 2 because if we draw out the structures given, there's no E1 rearrangement with H2SO4 to carbon #4 (if the alcohol were on C3). I would say that Experiment 2 is E2 because it gives a good amount of the least stable alkene, which doesn't usually occur with carbocations, but I think may occur with E2 pulling off an anti proton.

 

[SaintJude]

Thanks for trying this problem, b/c I definetely know that the most recent MCAT were similar to these Section Tests (I'm a retaker) But I don't think you're right about E2. I mean comparatively, to E1 we can see from the table there is more of the least stable alkene, but I think E2 produces the most stable alkene by default. The least stable alkene, likely occured from the deprotonation of 2 different H...

I don't know, I'll e-mail my Kaplan teacher...Can anyone help?

 

[syoung]

I chose A because of the answer choice: reaction with H2SO4 proceeds E1, while KOH proceeds E2.
H2SO4 is protic and polar, which will protonate the alcohol on the compound and cause it to leave, with a carbocation left behind. The KOH is a strong base and small, so will dehydrate the reaction into it's alkene E2, 1 step, no carbocation. CANNOT have carbocation because -OH is there, carbocation would never exist in basic solution.
The reaction takes place on C2.

 

[SaintJude]

I chose A because of the answer choice: reaction with H2SO4 proceeds E1, while KOH proceeds E2.
H2SO4 is protic and polar, which will protonate the alcohol on the compound and cause it to leave, with a carbocation left behind. The KOH is a strong base and small, so will dehydrate the reaction into it's alkene E2, 1 step, no carbocation. CANNOT have carbocation because -OH is there, carbocation would never exist in basic solution.
The reaction takes place on C2.

We are now watching the throne.

That was brilliant. Thank you! I did not realize that carbocations CANNOT occur in strongly basic solutions. It all makes sense now why strong bases favor E2 over E1...

Although, they carbocations occur with weak bases. Are you saying that's b/c weak, neutral bases, still allow the solution to be slightly acidic?

And I'm definetely going to start systematically looking at reagents first to hint whether it's E1 vs. E2

 

[piojita63]

I chose A because of the answer choice: reaction with H2SO4 proceeds E1, while KOH proceeds E2.
H2SO4 is protic and polar, which will protonate the alcohol on the compound and cause it to leave, with a carbocation left behind. The KOH is a strong base and small, so will dehydrate the reaction into it's alkene E2, 1 step, no carbocation. CANNOT have carbocation because -OH is there, carbocation would never exist in basic solution.
The reaction takes place on C2.

I agree with you. I would have chosen A since I have programmed in my brain that strong base= E2. Why is it C?? Or is it really A like we think? I am not familiar with that test format so I don't know if the boxed one is correct or the selected one....

 

[SaintJude]

The boxed one in green is correct

 

[syoung]

We are now watching the throne.

That was brilliant. Thank you! I did not realize that carbocations CANNOT occur in strongly basic solutions. It all makes sense now why strong bases favor E2 over E1...

Although, they carbocations occur with weak bases. Are you saying that's b/c weak, neutral bases, still allow the solution to be slightly acidic?

And I'm definetely going to start systematically looking at reagents first to hint whether it's E1 vs. E2

http://chemwiki.ucdavis.edu/Organic_Chemistry/Reactions/E1_Reaction
E1 can form a carbocation, if its secondary, AND if you use a weak base, and the reaction will deprotonate itself through solvolysis, rather than using the base to deprotonate, with heat.

Best thing for E1/SN1 is tertiary carbons