Structure and SN2 Reactivity The Nucleophile

Predict the relative basicities within each of the following groups, (a) OH, SH (b) PH2, SH (c) I , SeH (d) H0S02, H0S03 . Predict the relative acidities of the conjugate acids within each group. 

Now that we have looked at the effect of the leaving group, let us turn to a consideration of nucleophiles. How can we predict their relative nucleophilic strength, their nucleophilidty We shall see that nucleophilidty depends on a variety of factors charge, basicity, solvent, polarizability, and the nature of substituents. To grasp the relative importance of these effects, let us analyze the outcome of a series of comparative experiments. 

If the same nucleophilic atom is used, does charge play a role in the reactivity of a given nucleophile as determined by the rate of its Sn2 reaction The following experiments answer this question. 

Conclusion. Of a pair of nucleophiles containing the same reactive atom, the species with a negative charge is the more powerful nucleophile. Or, of a base and its conjugate acid, the base is always more nucleophilic. This finding is intuitively very reasonable. Because nucleophilic attack is characterized by the formation of a bond with an electrophilic carbon center, the more negative the attacking species, the faster the reaction should be. 

Predict which member in each of the following pairs is a better nucleophile, (a) HS or H2S (b) CH3SH or CHjS (c) CH3NH or CH3NH2 (d) HSe or H2Se. 

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