Where Does the α-Hydrogen Abstraction Occur in a Ketone-Ester Reaction?

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Discussion Overview

The discussion revolves around the mechanism of α-hydrogen abstraction in a ketone-ester reaction, specifically in the context of a Claisen condensation. Participants explore the role of ethoxide ion as a base and the implications of α-hydrogen abstraction from either the ketone or the ester.

Discussion Character

  • Homework-related
  • Debate/contested
  • Exploratory

Main Points Raised

  • One participant suggests that the ethoxide ion will abstract an α-hydrogen but is uncertain from which compound (ketone or ester) this occurs.
  • Another participant emphasizes the need to determine which α-hydrogen is more acidic before proceeding.
  • There is a proposal that the α-hydrogen abstraction occurs from the ester due to the formation of more resonating structures, though this is contested.
  • A participant expresses confusion about the mechanism, noting that using ethoxide to attack the carbonyl does not lead to a clear answer.
  • Some participants agree on the answer being C, indicating that the abstraction occurs from the ketone, but question why the ester is not preferred given its potential for resonance.
  • There is a discussion about the role of resonance structures and the inductive effect of the ethoxy group, with differing opinions on their significance.

Areas of Agreement / Disagreement

Participants generally do not reach a consensus on whether the α-hydrogen is abstracted from the ketone or the ester, with multiple competing views and ongoing debate regarding the implications of resonance and acidity.

Contextual Notes

Participants express uncertainty about the mechanism and the roles of resonance and acidity in determining the site of abstraction. There are references to various assumptions about the reactivity of the compounds involved, but these remain unresolved.

Saitama
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Homework Statement


(see attachment)


Homework Equations





The Attempt at a Solution


It looks to me a type of Claisen condensation but i did not realize it until i checked my book.
Before checking the book, i tried to do it by thinking on what is given in the question. We have a ethoxide ion here which acts as a base. It will probably abstract an α-hydrogen but from where? We have a ketone and an ester and both have acidic α-hydrogens. Please help me in understanding that from where the α-hydrogen is abstracted.

I have one more question (this may be a stupid one like the one above), can ethoxide ion directly attack carbonyl group present in any of the reactant? This would lead to breaking of double between carbon and oxygen towards oxygen but if i continue with this, i reach nowhere to the answer.
 

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Anyone?
 
I was taught that you should determine which alpha hydrogen is the most acidic.
 
If using NaOC2H5 to attack a carbonyl group gets you nowhere, you shouldn't do it. What is the 1st thing that happens in a Claisen Condensation mechanism?
 
chemisttree said:
I was taught that you should determine which alpha hydrogen is the most acidic.

What i think is that α-hydrogen gets abstracted from the ester because of more resonating structures formed after its abstraction. Is this right?
 
Running it with the ester as the reacting enolate gives me some weird molecule that I don't know what to do with. If only that ring was aromatic. Sorry I'm unable to be of assistance.
 
According to me, the answer should be C. I hope that to be right, considering I read Claisen Condensation just 5 minutes ago.
 
Indeed, the answer is C, the mechanism for Claisen condesation starts with the abstraction of alpha-hydrogen from ketone but this doesn't make sense to me, why not ester? More resonating structures are formed when hydrogen is abstracted from ester. :confused:
 
I got answer C also. Wouldn't that require using the carbonyl and methyl on the aliphatic ring to act as the enolate? Doesn't the aliphatic ring provide no resonance since each site has a full octet and cannot accommodate the lone electron pair on the ring?
 
  • #10
Pranav-Arora said:
More resonating structures are formed when hydrogen is abstracted from ester. :confused:

I don't think so. The oxygen in -O-Et is no way involved in resonance if your notions are as such. I followed the mechanism given at Wikipedia and got the answer.
 
  • #11
AGNuke said:
The oxygen in -O-Et is no way involved in resonance...

Why? :confused:
 
  • #12
Try drawing the resonance structure. There is no "significant" resonance, I should've added, at that O atom.
 
  • #13
AGNuke said:
Try drawing the resonance structure. There is no "significant" resonance, I should've added, at that O atom.

Agreed but what about the inductive effect of O in OEt? Doesn't that play any role?
 
  • #14
  • #15
chemisttree said:
Try this.

Thanks chemisttree, that's exactly what i am looking for. :smile:
 

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