Alcohol Dehydration?

[sfoksn]

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I was going through Kaplan material, and I found an interesting statement..

When 1-pentanol reacts with HCl with heat, it forms terminal alkene..

Is this true? I always thought alcohol dehydration would go E1, which means carbocation would form, which would lead to internal alkene in this case...

Can anyone confirm?

Thank you.

 

[whale86]

I was going through Kaplan material, and I found an interesting statement..

When 1-pentanol reacts with HCl with heat, it forms terminal alkene..

Is this true? I always thought alcohol dehydration would go E1, which means carbocation would form, which would lead to internal alkene in this case...

Can anyone confirm?

Thank you.

CH3CH2CH2CH2CH2OH + HCl --> CH3CH2CH2C=CH3

is this what your thinking? if what i drew is correct, that would be a terminal alkene.....correct me if im wrong...

 

[sfoksn]

Yea that is the terminal alkene, with 1 more hydrogen on the terminal carbon.

But why does it form that one, is my question.. Maybe I am just confused, but I thought the carbocation will rearrange to form the internal alkene.

 

[UndergradGuy7]

Yea that is the terminal alkene, with 1 more hydrogen on the terminal carbon.

But why does it form that one, is my question.. Maybe I am just confused, but I thought the carbocation will rearrange to form the internal alkene.

CH3-CH2-CH2-CH2-CH2OH

Right? If you do elimination. You protonate the OH to get OH2+ then you lose a H from the carbon in bold. So terminal alkene?

Since the OH is on a primary carbon (primary alcohol). Making a carbocation would not be possible and no rearrangements? So E2.

If the carbon was a secondary, then you could do E1.

 

[Baylor2011]

In addition, if it were to go under the E1 mechanism...the base would need to be dilute or very weak.

Also, wouldn't reaction w/ HCl/heat also produce a substitution product, like an alkyl chloride plus water? I'm not sure that HCl will induce elimination, however sulfuric acid is a different story. I know sulfuric acid and heat dehydrates alcohol...I'll look at my book when I get home.

 

[UndergradGuy7]

In addition, if it were to go under the E1 mechanism...the base would need to be dilute or very weak.

Also, wouldn't reaction w/ HCl/heat also produce a substitution product, like an alkyl chloride plus water? I'm not sure that HCl will induce elimination, however sulfuric acid is a different story. I know sulfuric acid and heat dehydrates alcohol...I'll look at my book when I get home.

I don't know specifically. But usually when I see something with heat. I assume elimination is the dominant not substitution. A substitution might happen, but much less than elimination because of the heat?

 

[Baylor2011]

That may be the trick...I see my organic professor later, I'll ask him! lol

 

[sfoksn]

alcohol dehydration is by E2 mechanism??
Wow I have been learning things the wrong way and my DAT is on Monday haha.

What other E2 mechanisms are there?

Thanks for your input guys

 

[UndergradGuy7]

alcohol dehydration is by E2 mechanism??
Wow I have been learning things the wrong way and my DAT is on Monday haha.

What other E2 mechanisms are there?

Thanks for your input guys

I went back to my orgo book. You were right. The reaction would not form a terminal alkene as the main product.

The example my book gives is butanol + H2SO4 and heat. It gives CH3CH=CHCH3 as the main product. Even though the alcohol is a primary, a hydride shift happens to make it secondary. Then it does E1, rearranges, and a terminal alkene is not formed as the major product.

But there are E1 and E2 alcohol eliminations. E2 is not as common, since there can't be anyway for the alcohol to rearrange.

Sorry for the incorrect info above.

 

[sfoksn]

Undergrad guy, would you mind giving me some examples where arrangements would happen and not happen?

Also when would it go E2 and when would it go E1?

Thanks 👍

 

[UndergradGuy7]

Undergrad guy, would you mind giving me some examples where arrangements would happen and not happen?

Also when would it go E2 and when would it go E1?

Thanks 👍

E1 if it can make a secondary or tertiary carbocation. If its a primary alcohol then E2.

The example for E2 they used is CH3-CH2OH. Even if you rearranged it is still primary. So E2. Most reactions are E1 I think.

I can't think of another since for dehydration of an alcohol you need the carbon next to the one with the OH to have a H. So it can always rearrange?

The other E2s they list are not for alcohols. They are for dehydrohalogenations of alkyl halides.

 

[Baylor2011]

To differentiate between E1 and E2:

E1 the base MUST be dilute or weak. If it's not either, it's E2

E2 must be in a nonpolar solvent. If it's polar, it's E1.

There's many more rules, but those are the really important ones.

 

[sfoksn]

If you rearranged the carbocation formed by H2O+ leaving onto the adjacent carbon, woudldn't that carbocation form secondary carbocation instead of original primary carboation?

I am confused why it won't rearrange.

 

[UndergradGuy7]

If you rearranged the carbocation formed by H2O+ leaving onto the adjacent carbon, woudldn't that carbocation form secondary carbocation instead of original primary carboation?

I am confused why it won't rearrange.

Which example?

In the original example you posted 1-pentanol reacts with HCl with heat. Yes, as I said in post above it does rearrange. In example in post 11 CH3-CH2OH then both are still primary.

 

[INH]

in terms of alcohol dehydrogenation (alcohol + acidic solvent), it is my general understanding the reaction mechanism will proceed E2 with a primary alcohol, and E1 for tert. & secondary alcohols

 

[dent777]

Why would there be a hydride shift for primary alcohol? Primary carbocations are not stable, shouldn't the reaction proceed via concerted (E2) mechanism where all the exchanges occur simultaneously. E1 involves a slow step, there is no slow step for primary carbocations.

P.S. According to Chad's video on alcohols, for primary alcohols the reaction mechanism is E2, therefore, I don't see how come there is a hydride shift and a more substituted alkene is formed.