Replies
Submitted by Matt
The best way to predict this is by the substitution of the alkyl halide, and remember that:
-SN1 reactions require carbocations (3° = good, 1° = bad)
-SN2 reactions don't form carbocations, and instead require a pentavalent, crowded transition state (3° = bad, 1° = good)
1° alkyl halides can't do SN1. 3° alkyl halides can't do SN2 (both both 1° and 3° alkyl halides can do E2)
2° can go either way. So the best way to tell for 2°'s is the strength of the nucleophile- look for negative charges. Nucleophiles like NaI, NaCN, KN3 etc. all have negative charges (I-, CN-, N3-) and so are at least decent nucleophiles, and will go SN2. Note that most of these compounds are basic.
Nucleophiles like H2O or ROH aren't charged, and so are usually weak nucleophiles, and tend to go SN1. A lot of the time these reactions are acid catalyzed.
So that's the general rule- basic conditions (negative charges) -> SN2 (or E2)
Acidic conditions (neutral or positive charges) -> SN1 (or E1)
See www.mendelset.com/problems/560 for a more detailed explanation. Also check out the SN1/SN2/E1/E2 review mendel set- www.mendelset.com/ms/908
Submitted by jeff
Substitution reactions are primarily affected by the substrate and solvent.
The SN2 mechanism proceeds through backside attack of the electrophile (or substrate) by the nucleophile. This occurs optimally when there is minimal substitution around the leaving group (1o >2o) because the nucleophile encounters less steric hindrance. A tertiary carbon can't undergo a SN2 reaction because surrounding carbons block this attack.
The SN1 mechanism proceeds through a carbocation intermediate. The leaving group departs, allowing the nucleophile to bond. If the SN2 is analogous to pushing some one off of a stool, the SN1 is like waiting for someone to get out of a lounge chair. The bar stool doesn't have much around it to protect the hapless individual you're pushing, making a backside attack relatively easy. But you can't push someone out of a lounge chair, because the back and arms help keep them seated. By analogy hindered substrates (3o>2o) will go via the SN1 mechanism.
That leaves solvent. The solvent molecules interact with your nucleophile and your substrate during the reaction. Polar protic solvents (e.g. water) will stabilize carbocation intermediates and nucleophiles. Polar aprotic solvents don't offer as much stabilization - carbocations can't form as easily and nucleophiles effectively become more reactive. Overall, an polar protic solvents favor an SN1 mechanism, while polar aprotic solvents favor the SN2.
