Substitution and Elimination Reactions of Primary Haloalkanes

Competing Substitution and Elimination Reactions of Primary Haloalkanes 

Primary haloalkanes can undergo either Sj 2 or E2 reactions. They do not undergo or El reactions because a primary carbocation is very unstable. Primary haloalkanes react with strongly nucleophihc, weakly basic reactants, such as ethyl thiolate (GHjCH S ), exclusively by an process. However, a primary haloalkane reacts with ethoxide ion, a weaker nucleophile, but a stronger base than ethyl thiolate, to give some elimination product. 

If a primary haloalkane is treated with ftrr-butoxide ion instead of ethoxide, the amount of ehmination product increases significantly. The fert-butoxide ion is not only more basic than the ethoxide ion also much more sterically hindered. The combination of these two factors favors elimination by an E2 process over substitution by an Sj 2 process. 

If the nucleophile is a weak base, primary haloalkanes react exclusively by an Sj 2 mechanism. 

If the nucleophile is a strong base that is nM sterically hindered, primary haloalkanes react by either an or an E2 mechanism, with the reaction predominating by a wide margin. 

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