fit2

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i have a question about this rxn because I seem to have found contradictory information. Question is simple : can rearrangements happen with dehydration of an alcohol. (when you add acid to an alcohol and an alkene is formed)

Thanks a lot for the clarification
 

orgoman22

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i have a question about this rxn because I seem to have found contradictory information. Question is simple : can rearrangements happen with dehydration of an alcohol. (when you add acid to an alcohol and an alkene is formed)

Thanks a lot for the clarification
Yes,,,indeed so. A secondary alcohol can dehydrate by the E1 mechanism if a more stable carbocation can form. A tertiary alcohol will RARELY dehydrate with a rearrangement unless a situation arises such as relief of ring strain. This would also be E1. Now,,,,the tricky part...Primary alcohols usually dehydrate by E2... since a primary carbocation does NOT form......HOWEVER.....if a primary alcohol has adjacent alkyl branches...then an E1-like mechanism operates where we first protonate the alcohol....and SIMULTANEOUSLY do a shift !!!! The Carey text and Dr. Paula Bruice text are one of the few texts that actually show this rare reaction . I hope this clears things up...if not...I will try to explain more

Dr. Romano
 
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fit2

fit2

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That makes sense. So rearrangement always happen if product formed in that case is more stable?
I understand the E1, E2 reactions from your explanation! Thank you!!!
 

ChewyDrop

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Defiantly can, I remember it has to do with the principle of microscope reversibility, how the mechanism of any reversible reaction must be the same but in reverse order. So.. the reverse of making an alcohol with an alkene, which follows markovnikov and carbocation rearrangement.
 

moose786

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Just to add my two cents, I recommend reading the Acid-Base chapter, as well as the substitution/elimination chapters in the Klein second language book. This will make you think conceptually in terms of mechanisms, and you will understand it rather than memorize. It will only save you time and silly errors in the future as well. He explains it really well so I don't think it will be too troubling!