Predicting Substitution vs. Elimination

In principle, any compound with a lone pair can act as either a base or a nucleophile toward C(sp3)-X, causing either E2 elimination or Sn2 substitution to occur. It is possible to predict whether substitution or elimination will occur with moderate accuracy (Table 2.1). Two factors largely determine the course of the reaction (1) the nucleophilicity and basicity of the lone-pair-bearing compound, and (2) the identity of the substrate Me or Bn, 1°, 2°, or 3° halide. 

The first factor that determines whether substitution or elimination occurs is the nucleophilicity and basicity of the lone-pair-bearing compound. The factors that influence nucleophilicity and basicity have already been discussed (Chapter 1). Nucleophiles-bases can be classified very simplistically into good nucleophiles-poor bases, good nucleophiles-good bases, and poor nucleophiles-good bases toward C(sp3) halides. 

Some common amidine and guanidine bases that are relatively nonnucleophilic  

Obviously the division of nucleophiles and bases into good and bad is a gross oversimplification. There is a continuum of nucleophilicity and basicity that runs from excellent through good, mediocre, poor, and awful. The simplistic classification given above merely provides a guideline for prediction of reactivity. 

The second factor that determines whether substitution or elimination occurs is the structure of the electrophile. 

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Table 2.1. Predicting substitution vs. elimination at C(sp3)-X under basic conditions...

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