Comparison of El and E2 Elimination Mechanisms

Let s summarize the major points to remember about the El and E2 reactions, focusing on the factors that help us predict which of these mechanisms will operate under a given set of experimental conditions. Then we will organize these factors into a short table. 

Effect of the Base The nature of the base is the single most important factor in determining whether an elimination will go by the El or E2 mechanism. If a strong base is present, the rate of the bimolecular reaction will be greater than the rate of ionization, and the E2 reaction will predominate (perhaps accompanied by the Sim2). 

If no strong base is present, with a good solvent a unimolecular ionization is likely, followed by loss of a proton to a weak base such as the solvent. Under these conditions, the E1 reaction usually predominates (always accompanied by the SnI). 

Effect of the Solvent The slow step of the El reaction is the formation of two ions. Like the 1, the E1 reaction critically depends on polar ionizing solvents such as water and the alcohols. 

In the E2 reaction, the transition state spreads out the negative charge of the base over the entire molecule. There is no more need for solvation in the E2 transition state than in the reactants. The E2 is therefore less sensitive to the solvent in fact, some reagents are stronger bases in less polar solvents. 

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