Factors Influencing the Course of Substitution Reactions

From the foregoing discussion it is clear that electron release or withdrawal will affect a substitution reaction differently, depending upon whether it is proceeding by preliminary dissociation or by the direct attack of an added reagent. (For example, electron release from It would hinder the approach of a negatively charged hydroxide ion (Sn%) while it would facilitate dissociation of the halide ion  

Thus in order to predict the effect of structure on reactivity, the process by which substitution occurs must be known. Often a careful study of reaction kinetics is the only way this question can be settled definitely.  

Unfortunately, however, for many organic reactions, such studies have not yet been carried out. Furthermore, while structural modifications usually do favor one process at the expense of the other, it is not always possible to classify a given substance as belonging unequivocally to the Sjfl or to the S 2 type. For a number of reactions it has been found that the substitution process may change from predominantly direct attack to predominantly preliminary ionization if the experimental conditions particularly favor this kind of reaction. The following factors of structure and environment might be considered. 

Solvent Effects in Nucleophilic Substitution (Data from Hughes, Trans Faraday Soc37, 009 1941]) 

Type and Mechanism Charges Concerned in Rate-Determining Stage of Reaction Effect on Charges of Forming Transition State Expected Effect of Polar Media on Rate 

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