Nucleophile substrate structure effects

The effects on SN2 reactions of the four variables—substrate structure, nucleophile, leaving group, and solvent—are summarized in the following statements and in the energy diagrams of Figure 11.7 ... 

In the discussion of electrophilic aromatic substitution (Chapter 11) equal attention was paid to the effect of substrate structure on reactivity (activation or deactivation) and on orientation. The question of orientation was important because in a typical substitution there are four or five hydrogens that could serve as leaving groups. This type of question is much less important for aromatic nucleophilic substitution, since in most cases there is only one potential leaving group in a molecule. Therefore attention is largely focused on the reactivity of one molecule compared with another and not on the comparison of the reactivity of different positions within the same molecule. 

The structure of the substrate influences the rate of reaction with a nucleophile, and this effect is reflected in the s values defined in the previous... 

Figure 10.20 Catalytic monoclonal antibody (MAb) mediated /3-elimination of HF. The MAb was generated through immunoreaction with the illustrated hapten linked to an appropriate carrier protein. The part of the hapten related to the substrate structure is shown in red. The tertiary amine functional group is introduced to encourage the generation of catalytic antibodies with complementary base/nucleophiles in the vicinity of the substrate a-proton when substrate binds to MAb in order to facilitate catalysis of E2 elimination by a combination of general base catalysis as well as the more usual differential stabilization of the rate determining step transition state through binding. Enzymes usually employ more than just differential stabilization of rate determining step transition states in order to effect catalysis but also employ several other physical "tricks" as well (see Chapter 8).

Examples of effects of substrate structure on the rates of nucleophilic substitution reactions have appeared in the preceding sections of this chapter. Additionally, some special effects will be covered in detail in succeeding sections. This section will emphasize the role steric effects can play in nucleophilic substitution reactions. 

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