Contrasting Elimination and Substitution

Almost all bases are also nucleophiles, and hence we expect competition between eliminations and substitutions. In both Sn2 and E2 reactions, the nucleophile or base reacts in a single rate-determining step with the reactant. In both SnI and El reactions, the nucleophile or base reacts in a step after the rate-determining heterolysis. Because the experimental observations for substitution and elimination reactions are so similar, we leave the discussion of kinetics to our discussion of substitutions in the next chapter. There are, however, some points that we should make about the factors that influence the extent of 5 2 versus E2 and SmI versus El reactions (Eq. 10.69). 

El reactions involve carbenium ion intermediates, and therefore are facilitated by all the factors that stabilize carbenium ions. These are the same factors that facilitate SnI reactions. Strongly ionizing solvents and substitution of electron donating groups on the carbon undergoing heterolysis are necessary. 

Percent Elimination Found for Various Alkyl Bromides in Two Different Solvents and with the Addition of the Very Weak Base Chloride or Strong Base Ethoxide  

Electron pushing for a specific example of an ElcB mechanism. 

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