[trainee]

I've always felt the "Oh, X is a sort of good leaving group so the nucleophile can attack in this environment" very unsatisfying. Is there something like a pka equivalent for LG?

[kriggy]

Well I dont know if there is anything like that but I think it could be possible but very difficult because there is a lot of different variables.

[Babcock_Hall]

I was surprised when I looked up tosylate versus bromide and iodide as leaving groups, to find out that their relative rates are very different in S_N1 versus S_N2 substitutions.

[pgk]

No, there is not.
Only empirical rules exist, which are based on experimental data.

[zsinger7]

As my mentor once told me, TRY IT!  In Sn2, the weaker base is usually the better LG I think.  (I.E. I- is a better LG than Cl- in DMSO)
        -Zack

[pgk]

Yes, but not exclusively.
Other factors, such as hard/soft basicity of the leaving group, also play an important role.
Indicatively, see a previous post:
”Advanced Organic Synthesis Problem”
http://www.chemicalforums.com/index.php?topic=92617.msg329905#msg329905