Organic Chem Question

[Guest karl]

Can someone tell me when SN1 rxns are favored over E1 rxns, and vice versa, and the same for SN2 rsxns vs. E2 rxns. I'd appreciate that. Thanks.

[Guest aneliz]

From my Orgo lecture notes I have the following:

 

SN1, E1 & E2 rxns favour 3>2>1 halides. This is due to the formation of a more stable carbocation for SN1 and E1 and due to the formation of a more stable/substituted alkene in E2 rxns.

 

SN2 rxns favour 1>2>3 halides as the steric hindrance is reduced.

 

In more detail:

 

Primary Halides react by the SN2 mechanism with the most basic nucleophiles producing the fastest rate of reaction. Therefore I<Br<Cl<F except in polar protic solvents (eg water) then the largest anion produces the fastest reaction rate. If there is much beta substitution then you may get an SN1 rxn.

 

Secondary Halides react by the SN2 mechanism with a good nucleophile but will react by the E2 mechanism with a strong base. Under solvolysis conditions, a secondary halide will react by the SN1 mechanism with some E1 reaction also occuring (minor).

 

Tertiary halides and sulfonates will react by the SN1 or E1 mechanism because they are able to form a stable carbocation intermediate. This reaction is fastest in polar protic solvents When tertiary halides are exposed to a strong base, E2 elimination will occur.

 

I hope that this helps you out. If you are still unclear, I would suggest getting your hands on an introductory organic chem text book. Good luck!