Trends in Nucleophilicity Within a Group

The order of nucleophilicity is opposite to the order of basicity for nucleophiles derived from atoms in the same group of the periodic table. First, consider the nucleophihcities of methane thiolate (CH3S ) and methoxide (CHjO ). Methane thiol is a stronger acid (pA = 10.6) than methanol (pAT = 15.5), and methoxide is therefore a stronger base than methane thiolate. However, methane thiolate is much more nucleophUic than methoxide (Table 10.1). The ratio of the relative rates for methylthiolate and methoxide in the displacement of iodide from methyl iodide is about 500 to 1. To react, a solvated nucleophile must lose some solvent molecules, so the nucleophile can approach the carbon center and start to form a bond to it. Therefore, its nucleophUicity is greatly decreased. 

The nucleophilicity of anions in a protic solvent such as an alcohol is diminished because of hydrogen bonding between the anion and the solvent. 

A similar inverse relationship between basicity and nucleophilicity occurs for the halides. Hydrogen iodide is a strong acid, and hydrogen fluoride is a weak acid. Thus, iodide ion is a weaker base than fluoride ion. However, for the series of halide ions, iodide ion is an excellent nucleophile and fluoride ion is a very poor nucleophile. The ratio of relative rates for iodide ion and fluoride ion in the displacement of iodide ion from iodomethane is about 50,000 to 1. 

The trend of anion nucleophilicity within a period of the periodic table is also affected by solvation. The larger, more polarizable anions are less strongly hydrogen bonded to protic solvents than smaller, less polarizable anions. As a result, less energy is required to shed solvent molecules from the larger ion to enable it react as a nucleophile. Thus, large anions are better nucleophiles than smaller anions because they are both more polarizable and less strongly solvated. 

Another way to describe nucleophilicity is to say that a highly polarizable nucleophile, which is likely to be a weak base, is a sofi nucleophile. Similarly, a base that is not very polarizable, which is likely to be a strong base, is a hard nucleophile. 

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