Question about ether cleavage by HBr

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In summary, the video discusses the process of a cyclic ether stealing a Hydrogen from HBr, resulting in a +1 formal charge and breaking the ring open. This helps alleviate the formal charge on the Carbon, which then bonds to the Bromide anion. The question arises as to why the oxygen would steal the Hydrogen in the first place, when it did not have a formal charge before. It is explained that the C-O bond breaks because of the nucleophilic attack of the bromide anion on the partial positive charge on the carbon. It is also mentioned that the protonated ether is in equilibrium with HBr and that eventually, a bromide will attack the alpha carbon to open the ring. However, it is noted
  • #1
CuriousBanker
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Sorry, just having one of those days were things don’t make sense to me
So in this video, this cyclic ether steals a Hydrogen off HBr. That gives it a +1 formal charge, so it breaks the ring open and steals the electrons from the Carbon to alleviate the formal charge. Carbon then has a formal charge which bonds to the Bromide anion that was formed from when it donated the Hydrogen to oxygen.
So my question is...if oxygen does not want to have a +1 formal charge and therefore needs to open the ring to alleviate it, why does it steal the hydrogen in the first place? It was a neutral atom before. Also, how is the oxygen in the ether more negative than a bromide ion which has a negative formal charge, enough to steal a hydrogen from anion?

 
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  • #2
I assume you’re talking about the part about 4:50 into the video (for the future, if you have a specific question about a 7 minute video, it’s a good idea to point people to the part of the video in question, so they don’t have to sit through the whole thing).

The C-O bond breaks because the nucleophilic bromide anion attacks the partial positive charge on the carbon.

The protonated ether is in equilibrium with the HBr. This step is (usually) fast, so at any given time, a certain fraction of ethers are protonated at the oxygen. The SN2 bromide attack on the alpha carbon is slower, so it’s not the case that every time an ether is protonated it undergoes ring cleavage. Eventually a bromide will be in the right place at the right time when the ether is protonated to attack the alpha carbon and this will open the ring.

(Also, if I’m being kind of pedantic, the formal reaction mechanism shouldn’t have a bunch of anionic bromide flying around in acidic conditions. Instead, the HBr should attack the alpha carbon directly to generate the SN2 product and a proton.)
 
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Related to Question about ether cleavage by HBr

1. How does HBr cleave ether?

HBr cleaves ether by attacking the oxygen atom of the ether molecule, forming a bromonium ion intermediate. This intermediate then undergoes a nucleophilic attack by the bromide ion, resulting in the formation of two alcohol molecules.

2. What is the mechanism of ether cleavage by HBr?

The mechanism of ether cleavage by HBr involves an acid-catalyzed nucleophilic substitution reaction, where HBr acts as both the acid and the nucleophile. The reaction proceeds through a bromonium ion intermediate and results in the formation of two alcohol molecules.

3. Can HBr cleave all types of ethers?

Yes, HBr can cleave all types of ethers, including simple ethers, cyclic ethers, and unsymmetrical ethers. However, the reaction may proceed at different rates depending on the structure and reactivity of the specific ether molecule.

4. What are the factors that affect the rate of ether cleavage by HBr?

The rate of ether cleavage by HBr can be affected by several factors, including the structure and reactivity of the ether molecule, the concentration of HBr, the presence of other functional groups, and the reaction temperature.

5. What are the main applications of ether cleavage by HBr?

Ether cleavage by HBr is commonly used in organic synthesis for the preparation of alcohols from ethers. It is also used in the industrial production of chemicals and pharmaceuticals. Additionally, this reaction can be used in the analysis and identification of ether-containing compounds.

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