radical reaction question

[jkelly30]

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I have a question about a reaction I encountered taking AAMC practice test 9. It's unrelated to any question in the passage, just a general reaction question.

Why doesn't any product form from the bromination of reactant "V"?

thanks 🙂

 

[CodeRedDew]

I'm not completely sure, but I believe it has something to do with the Br that is initially present in reactant II.

Since the reactants go through a radical intermediate, the intermediate that has an electronegative heteroatom near the newly formed radical is more stable due to field and inductive effects. I believe this is the reason why reactant II is more likely to react because it goes through the energetically lower intermediate. Not sure though, someone should chime in with a definite answer because mine are just speculations 😛

 

[osprey099]

I'm not completely sure, but I believe it has something to do with the Br that is initially present in reactant II.

Since the reactants go through a radical intermediate, the intermediate that has an electronegative heteroatom near the newly formed radical is more stable due to field and inductive effects. I believe this is the reason why reactant II is more likely to react because it goes through the energetically lower intermediate. Not sure though, someone should chime in with a definite answer because mine are just speculations 😛

Hmm I think the reaction between Br2 and IV and Br2 and V cannot occur because the radical intermediate will be too destabilized due to the hyperconjugation..

 

[CodeRedDew]

Hmm I think the reaction between Br2 and IV and Br2 and V cannot occur because the radical intermediate will be too destabilized due to the hyperconjugation..

Well, I don't think that's the case because the hyperconjugation that is going on between the reactant intermediates of IV and V are essentially the same. Hyperconjugation is defined as electron-donation via the bonds that are beta to the actual site that it's donating to. And if you look, the radicals that are formed are both "tertiary" radicals; therefore, the hyperconjugation that would be experienced by the radical site of both intermediates would basically be the same, so it's unlikely that one intermediate would be more destabilized by hyperconjugation.

 

[osprey099]

Well, I don't think that's the case because the hyperconjugation that is going on between the reactant intermediates of IV and V are essentially the same. Hyperconjugation is defined as electron-donation via the bonds that are beta to the actual site that it's donating to. And if you look, the radicals that are formed are both "tertiary" radicals; therefore, the hyperconjugation that would be experienced by the radical site of both intermediates would basically be the same, so it's unlikely that one intermediate would be more destabilized by hyperconjugation.

Yea that's what I meant (both reactions cannot occur due to too much destabilization). I think this is a bad question because of the stabilization issues..

 

[CodeRedDew]

Yea that's what I meant (both reactions cannot occur due to too much destabilization). I think this is a bad question because of the stabilization issues..

The reaction can definitely occur. Hyperconjugation doesn't destablize this reaction. A radical is an unpaired electron, so if anything, electron donation via the adjacent bonds should act to stabilize the unpaired electron. Which is also a reason why tertiary radicals are > secondary > primary.