MCAT Organic Chemistry Question

[Hopetocure2012]

When 1-butanol is dehydrated what is the product?

I say 2-butene (trans preferred, with cis and 1 butene as minor products)

The Princeton Review book says 1-butene is answer. (I am basing my question off of question one, passage 11 from the Organic Chemistry section in the Hyperlearning MCAT Science Workbook)

Any ideas?

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[Cool Beans]

If dehydrating in conc. H2SO4, the initial product (more of an intermediate as we'll see) formed is 1-butene. But being in a solution of H2SO4, 1-butene can be protonated and will be in equilibrium with the carbocation at carbon-2. When the carbocation undergoes elimination again, it preferentially goes to the more substituted (and more stable) 2-butene and as you stated the trans is preferred at that.

This one is tricky but this exact question was in my college textbook.

 

[SN2ed]

Thread moved. This type of thread belongs in the Study Q&A forum.

 

[Swagster]

I'd say 2-butene because you have to do it by E1 and you'll get rearrangement with acid. That seems like a simple problem that a prep company should get right.

 

[OneManShow]

If you look at the table
the enthalpy values are
1-butene: 30.3
cis-2-butene: 28.6
trans-2-butene: 27.6

I think 1-butene is the most stable in this case since it releases more energy? Not sure...

(By the way you can get to the answer, C, right away because B includes both the enthalpy values of the cis and trans 2-butene, but I guess you saw that already)

 

[GreenRabbit]

Too many people saying E1... H2SO4... This is a primary alcohol, E1 is not going to occur. Only E2 is going to occur and it will give you 1-butene.

In response to onemanshow, the enthalpy values show that 1-butene is the least stable. Even without using the table, you should know that alkylation stabilizes alkenes, so 1-butene should be less stable than 2-butene. Those enthalpy values don't represent energy released by dehydration.

 

[Jjkwest1]

Too many people saying E1... H2SO4... This is a primary alcohol, E1 is not going to occur. Only E2 is going to occur and it will give you 1-butene.

In response to onemanshow, the enthalpy values show that 1-butene is the least stable. Even without using the table, you should know that alkylation stabilizes alkenes, so 1-butene should be less stable than 2-butene. Those enthalpy values don't represent energy released by dehydration.

Even though it's primary, the overall product will be predominantly trans 2 butene because the 1 butene will pick up another proton and another elimination will occur.

 

[GreenRabbit]

Even though it's primary, the overall product will be predominantly trans 2 butene because the 1 butene will pick up another proton and another elimination will occur.

I don't have the passage he's referring to, it seemed to me like conc. H2SO4 was brought up in reference to some other textbook with "this exact question."

In either case, "When 1-butanol is dehydrated what is the product?" is a straightforward question, and should be answered with a straightforward answer. Valid point, but I think it's unwise to assume we're given enough time to find the most stable alkene.