dehydration

[dree]

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Correct me if I'm wrong
dehydration of 1-butanol yields 2-butene due to carbocation rearrangement?

I had a sample passage that said 1-butanol is dehydrated to 1-butene and now I'm confused.

 

[loveoforganic]

You'd probably get a mixture, but yeah, primarily 2 butene

 

[RogueUnicorn]

Correct me if I'm wrong
dehydration of 1-butanol yields 2-butene due to carbocation rearrangement?

I had a sample passage that said 1-butanol is dehydrated to 1-butene and now I'm confused.

are you sure the passage says that ONLY 1-butene is generated?

 

[dree]

Princeton Passage 11:

Q1: When 1-butanol is dehydrated, the new compound's heat of hydrogenation is most likely:
A. less than 27 kcal/mol
B. Between 27 and 30 kcal/mol
C. Between 30 and 33 kcal/mol
D. greater than 33 kcal/mol

answer: C) when 1-butanol is dehydrated, one obtains 1-butene, whose heat of hydrogenation is given in table 1 as 30.3 kcal/mol

**If it was 2-butene, the answer would have been B according to the table. Dehydration should form both products but shouldn't 2-butene be the answer since it is more stable?

 

[BerkReviewTeach]

Princeton Passage 11:

Q1: When 1-butanol is dehydrated, the new compound's heat of hydrogenation is most likely:
A. less than 27 kcal/mol
B. Between 27 and 30 kcal/mol
C. Between 30 and 33 kcal/mol
D. greater than 33 kcal/mol

answer: C) when 1-butanol is dehydrated, one obtains 1-butene, whose heat of hydrogenation is given in table 1 as 30.3 kcal/mol

**If it was 2-butene, the answer would have been B according to the table. Dehydration should form both products but shouldn't 2-butene be the answer since it is more stable?

There's a subtle question within a question here. With 1-butanol, even upon protonation of the hydroxyl group, it is highly unlikely that the water will leave carbon and form a primary carbocation. It's the same reasoning we use to say that primary carbons substitute exclusively by SN2 mechanisms and not the SN1 pathway.

This eimination (dehydration reaction) will likely procede by an E2 or E2-like pathway, resulting in 1-butene. I'm not sure if that is their reasoning as to why they chose the energy range of a terminal alkene rather than the internal alkene, but that would be my guess.

No matter what though, it sounds like you have such a good understanding of the topic that no matter whether it was an oversight on their part or a highly insightful answer, you get the concept and information quite well.

 

[docelh]

A few thoughts...

What are the possibilities of a concerted reaction pairing a hydride shift with a leaving water molecule? This could lead to a Zaitsev-compatible 2 degree carbocation.

Did the passage mention anything about the reagent being sterically hindered? If not, it's hard to see E2 beating out SN2.

 

[BerkReviewTeach]

Did the passage mention anything about the reagent being sterically hindered? If not, it's hard to see E2 beating out SN2.

I was wondering this same thing, but maybe the passage is asking about the minimal alkene side product. I have to believe they weren't really thinking at too much depth beyond losing an OH from C1 and H from C2, without stopping to think about the mechanism. I have a feeling we are all looking at this question at a depth beyond what the test writer was doing when they wrote it.

 

[loveoforganic]

Good catch BRT. It's still ambiguous though. You could 'force' and E1 mechanism with very high heat and concentrated acid, but there, you're going to go into the thermodynamic 2-butene. So which product you end up with depends on the conditions, which the question was nondescript about.