Solving Grignard Reagent Homework Problem

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Discussion Overview

The discussion revolves around a homework problem related to Grignard reagents and their application in organic synthesis, particularly involving the formation of allenes and cumulenes. Participants explore various mechanisms and propose different approaches to solve the problem.

Discussion Character

  • Homework-related
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant suggests that one of the bromine atoms should be replaced by magnesium bromide, but notes that the answer key indicates this is incorrect.
  • Another participant proposes writing the structure as Br--Mg--R- and questions what the answer key suggests, indicating that it involves an allene.
  • A participant mentions that the negative charge could attack a CH2 group, leading to the formation of CH2=C=C=CH2, which is a method to prepare cumulene.
  • There is a discussion about the mechanism involving a nucleophile attacking a carbon atom and the role of lone pairs, with a claim that this results in Butatriene.
  • One participant challenges the classification of the mechanism as SNi, stating that it is typically associated with specific reactions and suggests it may relate to neighboring group participation.
  • Another participant asserts that SNi is linked to various forms of neighboring group participation (NGP).
  • A request for references to support claims about the SNi mechanism is made, indicating a need for further validation of the arguments presented.

Areas of Agreement / Disagreement

Participants express differing views on the mechanisms involved and the classification of the reactions, particularly regarding the SNi mechanism and its applicability. There is no consensus on the correct approach to the homework problem or the validity of the proposed mechanisms.

Contextual Notes

Some participants express uncertainty about the mechanisms and terminology used, indicating a need for clarification and references to support their claims. The discussion includes assumptions about the structure and reactivity of the compounds involved.

Saitama
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Homework Statement


(see attachment)


Homework Equations





The Attempt at a Solution


One of Br should be replaced by MgBr but the answer key states that its wrong. I can't think of anything else. :confused:

Seems like I need to do something with alkyne.
Any help is appreciated.
 

Attachments

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Write it as Br--Mg--R-. Does R- rings a bell? What does the answer key suggests anyway?
 
AGNuke said:
Write it as Br--Mg--R-. Does R- rings a bell? What does the answer key suggests anyway?

Thanks AGNuke for the help! :smile:
The answer key suggests that its an allene.
If i write it as R-, I do get near to the answer but yet i can't seem to make the final compound. See attachment for my attempt. The negative charge and pi bonds form a conjugated system so there will be resonance.
 

Attachments

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What about that negative charge attacking CH2 and Br exiting? I am sure you would get CH2=C=C=CH2. As a matter of fact. It is a method to prepare otherwise hard to prepare cumulene.
 
Last edited:
AGNuke said:
What about that negative charge attacking CH2 and Br exiting? I am sure you would get CH2=C=C=CH2. As a matter of fact. It is a method to prepare otherwise hard to prepare cumulene.

I still don't get it. :rolleyes:

Care to explain a bit more.
 
Lets number the C-atoms in the diagram. The C-atom attached with Br-Mg be 1 and that last atom attached with Br be 4.

There are lone pair on C-3. They can act as a nuclophile and can attack C-4. On C-4, lone pairs from C-3 are coming and Br is going. This is SNi. And we get Butatriene.
 
AGNuke said:
Lets number the C-atoms in the diagram. The C-atom attached with Br-Mg be 1 and that last atom attached with Br be 4.

There are lone pair on C-3. They can act as a nuclophile and can attack C-4. On C-4, lone pairs from C-3 are coming and Br is going. This is SNi. And we get Butatriene.

How is it SNi? As far as I remember, this mechanism is found only during the reaction of R-OH with SOCl2 and there is one more reaction where this mechanism is found but it is definitely not related to what you have said. I think its related to Neighbouring Group participation but I would like to confirm it.
 
Substitution Nucleophilic Internal. It is also linked to many forms of NGP.
 
AGNuke said:
Substitution Nucleophilic Internal. It is also linked to many forms of NGP.

I know what's the meaning of SNi but I can't agree with you until you give me some references.
 
  • #10
Wikipedia?
 

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