When Do Nucleophilic Substitution and 3-Elimination Compete

Predict whether each /3-elimination reaction proceeds predominantly by an El or E2 mechanism, and write a structural formula for the major organic product  

Identify the solvent and the base. If the base is strong, an E2 mechanism is favored to occur. If the base is weak and the solvent is polar protic, then an El mechanism is favored to occur. 

Thus far, we have considered two types of reactions of haloalkanes nucleophilic substitution and 8-elimination. Many of the nucleophiles we have examined—for example, hydroxide ion and alkoxide ions—are also strong bases. Accordingly, nucleophilic 

Reactions of secondary and tertiary haloalkanes in polar protic solvents give mixtures of substitution and elimination products. In both reactions, Step 1 is the formation of a car-bocation intermediate. This step is then followed by either (1) the loss of a hydrogen to give an alkene (El) or (2) reaction with solvent to give a substitution product (Sfjl). In polar protic solvents, the products formed depend only on the structure of the particular carbocation. For example, tgrt-butyl chloride and t rt-butyl iodide in 80% aqueous ethanol both react with solvent, giving the same mixture of substitution and elimination products  

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