Why can't I form a double bond at the bridgehead in this nucleophilic substitution?

[Natalie456]

I have attempted this problem several times, but I keep on getting an incorrect answer. I have tried mostly substitution reactions (SN1 and SN2) on the top bromine atom, and I know that a double bond cannot be formed at the bridgehead. Any help in the right direction is greatly appreciated. Thanks!

 

[TeethWhitener]

What is the order of carbocation stability?

 

[TeethWhitener]

Also, given the geometry of the problem, is S_N1 or S_N2 more likely?

 

[Natalie456]

Would SN2 be more likely? I tried a substitution at the top bromine atom with the nucleophile -CN, but it was incorrect. I'm not sure where to go from there. For SN1, the carbocation at the top would be disubstituted. I'm assuming that the bromine at the bridgehead doesn't participate in any reactions.

 

[TeethWhitener]

I'm assuming that the bromine at the bridgehead doesn't participate in any reactions.

Why not?

 

[TeethWhitener]

Would SN2 be more likely?

Geometrically, does it look like a nucleophile will be able to attack the backside of either of the halogenated carbons?

 

[Natalie456]

I guess, geometrically, it doesn't look like it will be able to attack either from the backside. So it would be an SN1, then? Also, I thought there could be no reaction at the bridgehead because it cannot be planar, or it is too unstable. Is this correct?

 

[Natalie456]

I got it! Thank you so much for helping me reason through it!

 

[TeethWhitener]

Also, I thought there could be no reaction at the bridgehead because it cannot be planar, or it is too unstable. Is this correct?

Yes, that sounds right. Ignore where I was going before with that.

I guess, geometrically, it doesn't look like it will be able to attack either from the backside. So it would be an SN1, then?

This would be my guess.

 

[TeethWhitener]

I got it! Thank you so much for helping me reason through it!

Well, now I've thoroughly confused myself. Was it an Sn1 on the non-bridgehead? I need more caffeine.

 

[Natalie456]

Oh. Sorry. It was an Sn1 on the non-bridgehead!