Alkene Synthesis by Elimination of Alkyl Halides
When considering whether an elimination reaction is likely to occur via an E1 or E2 mechanism, we really need to consider three factors:
1) The base: strong bases favor the E2 mechanism, whereas, E1 mechanisms only require a weak base.
2) The solvent: good ionizing xolvents (polar protic) favor the E1 mechanism by stabilizing the carbocation intermediate.
3) The alkyl halide: primary alkyl halides have the only structure useful in distinguishing between the E2 and E1 pathways. Since primary carbocations do not form, only the E2 mechanism is possible.
|alkyl halide structure||tertiary > secondary > primary||tertiary > secondary >>>> primary|
|nucleophile||high concentration of a strong base||weak base|
|rate limiting step||anti-coplanar bimolecular transition state||carbocation formation|
|rate law||rate = k[R-X][Base]||rate = k[R-X]|
|stereochemisty||retained configuration||mixed configuration|
|solvent||not important||polar protic|