Sn1 Reactions: Substrate structure vs leaving group stabilit

[ThousandFjord]

Homework Statement

Let's say I have two compounds in an identical solvent. The compounds are also identical except for the following: One has a Br bonded to a tertiary carbon, and the other has an I bonded to a secondary carbon. Which would react first in an Sn1 nucleophilic reaction?

Homework Equations

N/A

The Attempt at a Solution

On the one hand, the first compound would form a more stable intermediate (tertiary vs secondary carbon), but on the other hand, the second compound has a more stable leaving group (I vs Br). So I have no idea. Basically, I'd like to know which is more important in Sn1 reactions: substrate structure or leaving group stability? Why? (And where would solvent polarity fit in the ranking, while we're at it?)

 

[epenguin]

Homework Statement

Let's say I have two compounds in an identical solvent. The compounds are also identical except for the following: One has a Br bonded to a tertiary carbon, and the other has an I bonded to a secondary carbon. Which would react first in an Sn1 nucleophilic reaction?

Homework Equations

N/A

The Attempt at a Solution

On the one hand, the first compound would form a more stable intermediate (tertiary vs secondary carbon), but on the other hand, the second compound has a more stable leaving group (I vs Br). So I have no idea. Basically, I'd like to know which is more important in Sn1 reactions: substrate structure or leaving group stability? Why? (And where would solvent polarity fit in the ranking, while we're at it?)

You have two factors, one favours one reaction, the other favours the other. I don't know what other knowledge one can draw on to say which is more important. But if there is something like solvent that influences one of these factors mainly you could make a prediction about how it affects the relative rates of these two. The lecture below says something about that towards the end.

http://www.khanacademy.org/science/organic-chemistry/substitution-elimination-reactions/sn1-sn2-sal/v/solvent-effects-on-sn1-and-sn2-reactions

and there seem to be other resources on the web.