10-17-08 - 318M Krische, Lecture 22: F - 10/17/08,...

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318M – Krische, Lecture 22: F - 10/17/08, Substitution/Elimination Having determined the mechanism of the S N 2 reaction, we now discuss the factors that are important for promoting the S N 2 reaction. The most important factors are: (1) Nucleophile Structure, (2) Electrophile Structure and (3) Solvation. Nucleophile Structure : When all else is equal, electrons that reside on an electropositive element are more nucleophilic than those that reside on an electronegative element. Thus, we predict the following trend: within a given period (row) nucleophilicity parallels basicity. For a given element, anionic species are better nucleophiles than corresponding neutral species. For example, sodium acetate is a far better nucleophile than acetic acid. For a given element, nucleophilicity increases with increasing basicity. For example, sodium ethoxide (pKa of ethanol = ~16) is a better nucleophile than sodium CF 3 CH 2 ONa (pKa of CF 3 CH 2 OH = 12). In other words, the less stable the nucleophilic anion, the
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