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Concerning alcohol dehydration, I get that it's E1 in secondary and tertiary alcohols, that it's E2 in primary alcohols, and also why E1 reaction is easier for tertiary alcohols as compared to secondary ones.

Why is the ease of dehydration for primary alcohols less than both of these? It would have been obvious it was reacting through E1, but it's doing E2, which is favourable for primary alcohols.

Why then, is it harder to dehydrate primary alcohols than either of the other two?

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    $\begingroup$ Because most dehydration conditions are geared towards E1. That is, the acidic conditions create an oxonium ion that is more amenable to generating a carbocation, but a primary alcohol isn't very likely to generate the cation. $\endgroup$
    – Zhe
    Apr 26, 2021 at 2:50
  • $\begingroup$ @Zhe: oh, ok, so E2 requires strong bases, but the medium's acidic and that favours E1. Is that how it works? $\endgroup$
    – harry
    Apr 26, 2021 at 3:00
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    $\begingroup$ That's more or less how I'm seeing it. $\endgroup$
    – Zhe
    Apr 26, 2021 at 12:54

2 Answers 2

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The rate of reaction depends on how stable the transition state is. In both E1 and E2 good leaving group is present along with presence of a weak and strong base ,respectively. This leads to an alkene type of transition state in E2 mechanism and a cationic type transition state in E1 mechanism . Both these transition states gets stabilized by the electron donating ability of the attached alkyl groups (i.e By Hyperconjugation and Inductive Effect).This leads to the increase in rate of reaction and thus the secondary and tertiary alcohols are easier to dehydrate than the primary alcohols. Hope this helps!!

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Acid catalysed dehydration of primary alcohols is primarily by an E1 mechanism hence slower than dehydration of secondary and tertiary alcohols. The competing reaction is SN2 formation of ethers by attack of an alcohol molecule on the hydrated molecule. Bidentate reactions such as the Oppenauer oxidation, oxidation by quinones, or even chromate oxidation result in alpha attack and give ketones or aldehydes. The various mechanisms could possibly involve beta abstraction to form enolates with subsequent isomerization. I don't know of a direct E2 elimination to give alkenes involving alcohols in acidic solutions.

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