Solving Grignard Reagent Homework Problem

[Saitama]

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.

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

 

[AGNuke]

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

 

[Saitama]

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

Thanks AGNuke for the help!
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.

 

[AGNuke]

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.

 

[Saitama]

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.

Care to explain a bit more.

 

[AGNuke]

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 S_Ni. And we get Butatriene.

 

[Saitama]

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 S_Ni. And we get Butatriene.

How is it S_Ni? 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.

 

[AGNuke]

Substitution Nucleophilic Internal. It is also linked to many forms of NGP.

 

[Saitama]

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.

 

[AGNuke]

Wikipedia?