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Ch_322a_6.018 - Solvent Effects on SN2 Reactions The choice...

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Solvent Effects on S N 2 Reactions The choice of solvent greatly influences the rate of nucleophilic substitution reactions. Important solvent properties are polar/nonpolar and protic/aprotic . Often a solvent is classified as polar protic, polar aprotic, or nonpolar. Polar Protic Solvents Protic solvents have hydrogens attached to highly electronegative atoms, such as O and N, that form hydrogen bonds to nucleophiles. Protic solvents are also polar solvents and are often called polar protic solvents . O R H !" ! + Water and alcohols are polar protic solvents. Halide ions interact with polar protic solvents through hydrogen bonding: X: : : : H O R H O R H H O O R R -
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Nucleophile Stability and Reactivity The strength of the H-bonding increases with the density of the negative charge on X - . Fluoride ion , the smallest halide, has the largest negative charge densityand strongest H-bonding interaction with polar protic solvents. This interaction stabilizes the ion and decreases its nucleophile strength in polar protic solvents.
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