Question about reactivity of substituted furan in H3O

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Discussion Overview

The discussion revolves around the reactivity of a substituted furan compound in acidic conditions (H3O). Participants explore the behavior of methoxy and dimethyl ether groups in this context, particularly focusing on potential reactions such as acetal formation and the mechanisms involved.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Angela describes a compound resembling tetramethoxyfuran, questioning the reactivity of its methoxy groups in acidic solution.
  • One participant introduces the concept of acetals and their distinct reactivity in acid compared to ethers.
  • Angela reflects on the differences between acetals and ethers, considering the electron-withdrawing effect of multiple oxygen atoms on reactivity.
  • Another participant suggests that the reaction can proceed through multiple pathways, including acetal formation and hemi-acetal formation, and discusses the potential mechanisms involved.
  • Angela expresses confusion regarding the formation of an oxocarbenium ion and its relation to the proposed mechanisms, seeking clarification on its nature.
  • A participant explains the concept of oxocarbenium ions, highlighting their stability compared to regular carbocations and their role in nucleophilic attack.
  • Angela acknowledges the explanation and indicates a desire to further understand the concepts through visual representation.

Areas of Agreement / Disagreement

Participants demonstrate varying levels of understanding regarding the reactivity of the compound in question, with some agreeing on the role of acetals and oxocarbenium ions while others express uncertainty about the mechanisms and pathways involved. The discussion remains unresolved with multiple competing views on the reactivity and mechanisms.

Contextual Notes

Participants reference specific reactions and mechanisms without consensus on the exact pathways or the implications of the proposed models. There is an acknowledgment of the complexity of the reactions and the need for further clarification on certain concepts.

amb123
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My textbook doesn't discuss this and the ACS exam study guide asks about it.

I don't know how to name this, but basically it is almost a tetramethoxyfuran, except that one of the methoxy groups is actually dimethyl ether (methylmethoxy?) So, I'm going to take a stab at this and call is 1-methylmethoxy-2,3,4-methoxy furan. I hope this is correct or at least nearly correct.

Anyhow, in acidic solution (H3O) what happens? The book says that only one of the methoxy groups reacts to give a hydroxy group there, the other two and the methylmethoxy remain unreacted. I thought that any ether (ROR) needed more extreme conditions to react. Obviously I am mistaken. Can someone explain?

Thanks!
Angela.
 
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Are you familiar with acetals? How do acetals react in the presence of acid? What makes them different from a regular ether?
 
I thought acetal when I first saw it but then it looked weird so I wasn't 100% on it and then thought of reactions of ethers. I definitely needed some acetal review, and I've spent the last 20 minutes going through the reactions that would create this compound, and that would hydrate it.

Oh, and on the questions of why acetals react differently, is it because of the electron withdrawing affect of two oxygen atoms instead of one makes the carbon much more reactive toward attack by a nucleophile (H2O)?

Thanks for the help!
Angela.
 
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I think of the difference between acetals and ethers as in both cases you have a decent leaving group (alkoxide) but with an acetal you have the lone pairs from the other oxygen to assist in kicking out the leaving group (making an oxocarbenium ion).

Does that help any?
 
I am seeing that this reaction can go one of three ways. It can go acetal. It can go hemi-acetal through cleavage of the ring oxygen, or it can go hemi-acetal by cleavage of the -OCH3 group in question. Protonation of the -OCH3 group makes it a good leaving group, If it were SN1 the methanol would leave a carbocation adjacent to the ring oxygen. I am not really sure how this works because i'd think that the ringed oxygen pulled electron density away from the adjacent carbon making it impossible to give a carbocation. Maybe this is then SN2, and the carbon adjacent to the ring oxygen is then a good candidate for SN2 nuclephilic attack (by H2O) due to it being very positive. That is all I could come up with to explain this reaction.

What is oxocarbenium? Is that an anion where H+ or +OR group leaves electrons behind or something? My book has no reference to this can you explain what it is and what mechanism you are proposing here? I am confused.

Thanks again!
-A
 
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Nucleophilic attack on oxygen is almost always a bad thing. There are very few reagents which permit this.

An oxocarbenium ion is a resonance structure of for a carbocation adjacent to an oxygen atom. In this resonance structure, a lone pair from the oxygen has formed a double bond with the carbon so that the oxygen bears a formal positive charge. This is actually a reasonably good resonance structure. Like carbonyl groups, nucleophiles will attack oxocarbenium ions at carbon to ultimately quench the positive charge.

They best way to think of why oxocarbenium ions relative to regular carbocations is that you can draw another reasonable resonance structure and therefore oxocarbenium ions are actually lower in energy than unstabilized carbocations!

Does this help your understanding?
 
Yes, I can see what you are saying. Let me play around with it and see if I can explain it to myself through drawings. Thanks so much!

:)
Angela.