[LTK]

What is unique about this alkyl halide (1-Bromobicyclo[1.2.2]heptane) that prohibits a reaction of both Sn1 & Sn2?
I know that Sn2 will not occur due to the Br being attached to a tertiary carbon, but I'm not sure about the Sn1 reaction.

[orgoclear]

Think about the intermediate that would be formed if the reaction was able to proceed through an SN 1 pathway. (think of the structure of the carbocation). And yes SN 2 is not viable because the alkyl halide is tertiary

[dunno260]

The resulting alkene would violate Bredt's rule because the resulting strain from the trans-alkene you would have in the ring would be enormous.  

http://en.wikipedia.org/wiki/Bredt%27s_rule

[LTK]

That's what I was leaning toward because of the steric hinderance. So Bredt's rule would apply regardless of any reactant that's introduced, such as AgNO3/EtOH? Thanks a lot for the help. 

[Jorriss]

That's what I was leaning toward because of the steric hinderance. So Bredt's rule would apply regardless of any reactant that's introduced, such as AgNO3/EtOH? Thanks a lot for the help. 

I am not familiar with Bredt's rule but I don't believe the inhibiting factor for an Sn1 reaction is steric hindrance. I would lean towards a hybridization reason.

[LTK]

That's what I was leaning toward because of the steric hinderance. So Bredt's rule would apply regardless of any reactant that's introduced, such as AgNO3/EtOH? Thanks a lot for the help. 

I am not familiar with Bredt's rule but I don't believe the inhibiting factor for an Sn1 reaction is steric hindrance. I would lean towards a hybridization reason.

Could you elaborate a little more?

[Jorriss]

That's what I was leaning toward because of the steric hinderance. So Bredt's rule would apply regardless of any reactant that's introduced, such as AgNO3/EtOH? Thanks a lot for the help. 

I am not familiar with Bredt's rule but I don't believe the inhibiting factor for an Sn1 reaction is steric hindrance. I would lean towards a hybridization reason.

Could you elaborate a little more?

Think in terms of the bond angles and the hybridization in an Sn1 reaction. What are the bond angles, ideal and actual, of your starting compound? What are the bond angles and hybridization for your carbocation intermediate?

[LTK]

109.5 vs 120?

[Jorriss]

109.5 vs 120?

Those are the ideal angles.

But with a caged or bicyclic compound, the angles are locked in. So your starting compound and your carbocation intermediate will have approx. the same angles.

Before you some sp3 hybridized carbons with bond angles of < 109 degrees. Afterwards we have an sp2 hybridized carbon with bond angles that are still less than 109 degrees. Meaning, the sn1 route will increase the amount of angle strain on the molecule.

That is my line of thinking on this problem.

[SVXX]

Jorriss is right. Basically the intermediates which form in S_N1, S_N2, E1 and E2 are sp² hybridized or planar. The bridgehead carbon(where the bromine or leaving group is located) has considerable angle strain which prevents it from becoming planar at all. So none of these reactions will work at the bridgehead carbon of the bicyclic compound.

[LTK]

Thanks