[jt0608]

Hi forum, I need help with a problem regarding solvolysis rate.
(The number on the bottom indicates the solvolysis rate.)


From what I understand the solvolysis rate is determine by how stable the carbocation is after -Cl leaves. What I'm having trouble understanding is why 1 is much more faster than 4 even though both are tertiary carbocation. And why 5 is slower than 4 when the additional sigma bond is there.

Any insights would be really helpful.
Thank you

[sjb]

What is the geometry of a carbocation?

[jt0608]

What is the geometry of a carbocation?

sp2 trigonal planar. Could it be that the empty p-orbital lies inside the ring, making it harder for the solvent to attack due to steric hindrance?

[Dan]

sp2 trigonal planar.

Get you model kit out and try to build the bicyclic carbocation with an sp² at the bridgehead - you will see the problem.

[jt0608]

Get you model kit out and try to build the bicyclic carbocation with an sp² at the bridgehead - you will see the problem.

Oh, I think I see it now. I don't have a kit with me at the moment, but from what you say and looking at the problem, the cation at the bridgehead isn't planar, so the angle strain then would play a part in its stability and the overall solvolysis rate ?

Not sure if what I stated is correct, but it makes a lot more sense than my old explanation with anchimeric assistance.

[georginaketh]

I always refer the following site for such chemistry problems. You can go through this link once and see if you can get the answer.
http://pubs.acs.org/doi/abs/10.1021/ja01182a117


(removal of errant link, sjb)