Nucleophilicity and Solvent Effects

The term nucleophilicity refers to the effect of a Lewis base on the rate of a nucleophilic substitution reaction and may be contrasted with basicity, which is defined in terms of the position of an equilibrium reaction with a proton or some other acid. Nucleophilicity is used to describe trends in the kinetic aspects of substitution reactions. The relative nucleophilicity of a given species may be different toward various reactants, and it has not been possible to devise an absolute scale of nucleophilicity. We need to gain some impression of the structural features that govern nucleophilicity and to understand the relationship between nucleophilicity and basicity.  

Many properties have an influence on nucleophilicity. Those considered to be most significant are (1) the solvation energy of the nucleophile (2) the strength of the bond being formed to carbon (3) the size of the nucleophile (4) flie electronegativity of the attacking atom and (5) the polarizability of the attacking atom. Let us consider how each of these factors affects nucleophilicity  

A stronger bond between the nucleophilic atom and carbon is reflected in a more stable transition state and therefore a reduced activation energy. Since the 8 2 process is concerted, the strength of the partially formed new bond is reflected in the energy of the transition state. 

A sterically restricted nucleophile is less reactive than a more accessible one because of nonbonded repulsions which develop in the transition state. The trigonal bipyramidal geometry of the 8 2 transition state is sterically more demanding than the tetrahedral reactant, so steric congestion increases as the transition state is approached. 

For general reviews of nucleophilicity, see R. F. Hudson, in Chemical Reactivity and Reaction Paths, G. Klopman, ed., John Wiley Sons, New York, 1974, Chapter 5 J. M. Harris and S. P. McManus, eds., Nucleophilicity, Advances in Chemistry Series, fio. 215, American Chemical Society, lA asbingtuo, D.C., 1987. 

The factors that influence nucleophilicity have usually been assessed in the context of the limiting Sn2 case, since it is here that the properties of the nucleophile will be most apparent. The rate of an Sn2 reaction is directly related to the effectiveness of the nucleophile in displacing the leaving group. In contrast, the effect of nucleophilicity will not be evident in the rate of an SnI reaction. The nature of the nucleophile will only affect the product distribution resulting from partitioning of the carbocation intermediate among the available pathways. 

Streitwieser, Jr., Solvolytic Displacement Reactions, McGraw-Hill, New York, 1962 J. F. Bunnett, Annu. Rev. Phys. Chem. 14, 271 (1963). 

Streitwieser, Jr., Solvolytic Displacement Reactions, McGraw-Hill, New York, 1962. 

Empirical measures of nucleophilicity may be obtained quite readily by comparing relative rates of reaction of a standard substrate with various nucleophiles. Rather than tabulate rate constant data, it has become customary to express this property in terms of a nucleophilic constant (n). Swain and Scott proposed to correlate solvolysis rates by the equation 

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Figure 11.7 Energy diagrams showing the effects of (a) substrate, (b) nucleophile, (c) leaving group, and (d) solvent on Sn2 reaction rates. Substrate and leaving group effects are felt primarily in the transition state. Nucleophile and solvent effects are felt primarily in the reactant ground state.

Before we attempt to apply this approach to specific cases, let s have a look at the guidelines summarized in Figure 27-13. The guidelines are presented in the form of a decision tree, with the first consideration being the base strength of the electron pair donor, B. Additional considerations include steric hindrance, nucleophilicity, and solvent effects. 

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