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Note: Primary alcohol will follow E2 mechanism, therefore, removal of protonated water and proton (RDS) shall proceed in a single step in the above mechanism.

My questions are:

1- According to wikipedia "Although the parent compound has not been isolated, some substituted derivatives are robust and a single molecule of cyclobutadiene is quite stable."

As cyclobutadiene is not a stable compound, Is the primary alcohol (reactant) a stable compound?

2- Is the ring expansion step and the final product (benzene) correct?

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  • $\begingroup$ Your note doesn't make sense. An E2 elimination would immediately lead to an alkene with no intermediates that could react via side pathways. The ring expansion (aside from the issues of arrow pushing described in one of the answers) is extremely unusual. Do you know of a literature precedent for such a mechanism? $\endgroup$ – Zhe Aug 6 at 17:37
  • $\begingroup$ Some context would help. Are you proposing a mechanism for a known reaction? If so, do you have a reference or at least conditions? Or you are suggesting a transformation? $\endgroup$ – Zhe Aug 6 at 17:52
  • $\begingroup$ @Zhe (1) So, is ring expansion not possible in E2 mechanism because elimination of protonated water and β H occur in a single step? (2) I have proposed this reaction for this unstable alcohol; it's not a known mechanism. $\endgroup$ – Apurvium Aug 6 at 18:19
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    $\begingroup$ (1) Yes. (2) In that case, my opinion of the ring expansion being unusual has changed to it being highly unlikely. Note that that compound is unstable merely because it has a cyclobutadiene, which will dimerize via Diels-Alder. $\endgroup$ – Zhe Aug 6 at 18:44
  • $\begingroup$ @zhe So, the product will be 1-ethylidene cyclobut-1,3-diene which may be stable due to conjugation? $\endgroup$ – Apurvium Aug 6 at 19:14
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You have misinterpreted what Wikipedia has said. I'd put whole quote here:

Cyclobutadiene is an organic compound with the formula $\ce{C4H4}$. It is very reactive owing to its tendency to dimerize. Although the parent compound has not been isolated, some substituted derivatives are robust and a single molecule of cyclobutadiene is quite stable. Since the compound degrades by a bimolecular process, the species can be observed by matrix isolation techniques at temperatures below $\pu{35 K}$. It is thought to adopt a rectangular structure.

These somewhat stable cyclobutadiene derivatives are not just derivatives with a simple alkyl substituent. They are with bulky substituents, e.g., tetrakis(tert-butyl)cyclobutadiene. Therefore it is safe to assume that given compound (2-cyclobutadienylethanol) does not exist. However, as Aniruddha Deb has said, we can treat it like it is existing only for this theoretical question (we don't have other choice):

Since it is a primary alcohol it can't undergo usual $\mathrm{E1}$ elimination with an acid. The $\mathrm{E1}$ pathway through formation of a primary carbocation is not the most likely pathway here, because primary carbocations are extremely unstable. Therefore, more likely, this reaction undergoes through an $\mathrm{E2}$ mechanism where the transition state should be lower in energy. Accordingly the protonation of the alcohol group to give the good leaving group $\ce{-OH2+}$:

Protonation of 2-Cyclobutadienylethanol

And then, a weak base (which could be either $\ce{H2O}$ or $\ce{H2PO4-}$, or even another molecule of the alcohol) could abstract $\beta-\ce{H}$ from nearby $\beta$-$\ce{CH2}$ group, leading to give the conjugated alkene with cyclobutadienyl group (vinylcyclobutadiene):

Rearrangement to benzene

We really don't know how stable vinylcyclobutadiene is, but it should be more stable than original alcohol since it has additional resonance contribution with external vinyl group. This compound can undergoes either Diels-Alder or Cope rearrangement to give a bicyclo[2.2.0]hex-1,5-diene derivative. This derivative should be unstable due to severe strain, and expected to rearrange to Dewar benzene derivative, which then rearrange to benzene upon heating (Ref.1).

Reference:

  1. L. Watts, J. D. Fitzpatrick, R. Pettit, "Cyclobutadiene," J. Am. Chem. Soc. 1965, 87(14), 3253–3254 (https://doi.org/10.1021/ja01092a049).
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  1. In real-life, Cyclobutadiene dimerizes at temperatures above 35 K by a Diels-Alder reaction (source: Wikipedia). However, this seems more like a theoretical reaction rather than a practical one, so treat it as such and stick to the task of finding the dehydration product of this cyclobutadiene derivative.
  2. Yes, this step is correct (although the arrow-pushing mechanism seems a bit wrong). Benzene has a large resonance energy and the dehydration would favour the formation of Benzene. This is another question on similar lines, where toluene is formed by the dehydration of (cyclobutane-1,2,3-triyl)trimethanol.
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  • $\begingroup$ +1, Can you please point out where the arrow-pushing went wrong? $\endgroup$ – Apurvium Aug 6 at 10:12
  • $\begingroup$ arrow pushing doesn't show thr bond being formed between (1) and (6) $\endgroup$ – Aniruddha Deb Aug 6 at 17:34
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    $\begingroup$ The linked mechanism is very different from what is being posited here. $\endgroup$ – Zhe Aug 6 at 17:36
  • $\begingroup$ @Zhe linked mechanism explains how benzene is formed from a smaller ring by successive ring expansion. Its not a point-to-point answer for this question, just an example that may be helpful. $\endgroup$ – Aniruddha Deb Aug 6 at 17:38
  • $\begingroup$ But the linked example is a 1,2-shift, which is well-known in the literature. The example here is a 1,3-shift (?) coupled with a hydride migration that is highly unusual. $\endgroup$ – Zhe Aug 6 at 17:50

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