Question about reactivity of substituted furan in H3O

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In summary, acetals are different from regular ethers in that they have a lone pair of electrons from the other oxygen atom to assist in kicking out the leaving group (making an oxocarbenium ion). This reaction can go one of three ways- acetal, hemi-acetal through cleavage of the ring oxygen, or hemi-acetal by cleavage of the -OCH3 group. 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.
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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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  • #2
Are you familiar with acetals? How do acetals react in the presence of acid? What makes them different from a regular ether?
 
  • #3
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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  • #4
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?
 
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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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  • #6
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?
 
  • #7
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.
 

What is furan and why is it important in chemistry?

Furan is a heterocyclic compound with a five-membered ring containing four carbon atoms and one oxygen atom. It is important in chemistry because it is a common building block for many natural and synthetic compounds, and its reactivity and versatility make it useful in various chemical reactions.

How does the substitution of furan affect its reactivity in H3O?

The substitution of furan with different functional groups can significantly impact its reactivity in H3O. For example, electron-donating substituents such as -OH or -NH2 increase the electron density on the furan ring, making it more nucleophilic and prone to attack by H3O+. On the other hand, electron-withdrawing substituents like -COOH or -NO2 decrease the electron density, making furan less reactive towards H3O+.

What factors influence the reactivity of substituted furan in H3O?

The reactivity of substituted furan in H3O is influenced by several factors such as the nature and position of the substituent, the solvent used, and the reaction conditions (temperature, pH, etc.). Additionally, steric hindrance, resonance effects, and inductive effects can also impact the reactivity of substituted furan in H3O.

What are some common reactions of substituted furan in H3O?

Furan derivatives can undergo various types of reactions with H3O+ depending on the nature of the substituent. Some common reactions include electrophilic aromatic substitution, nucleophilic addition, and protonation. These reactions can lead to the formation of different products such as furanols, furanones, or furan carboxylic acids.

How is the reactivity of substituted furan in H3O studied in the laboratory?

In the laboratory, the reactivity of substituted furan in H3O can be studied using various techniques such as spectroscopy, chromatography, and NMR. These methods allow scientists to analyze the products formed and determine the rate of the reaction, as well as explore the different factors that influence the reactivity of substituted furan in H3O.

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