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Date: 27-11-2019
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Date: 8-7-2018
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SN2 reactions-The Solvent
The rates of SN2 reactions are strongly affected by the solvent. Protic solvents— those that contain an - OH or - NH group—are generally the worst for SN2 reactions, while polar aprotic solvents, which are polar but don’t have an - OH
or - NH group, are the best. Protic solvents, such as methanol and ethanol, slow down SN2 reactions by solvation of the reactant nucleophile. The solvent molecules hydrogenbond to the nucleophile and form a cage around it, thereby lowering its energy and reactivity.
In contrast with protic solvents—which decrease the rates of SN2 reactions by lowering the ground-state energy of the nucleophile—polar aprotic solvents increase the rates of SN2 reactions by raising the ground-state energy of the nucleophile. Acetonitrile (CH3CN), dimethylformamide [(CH3)2NCHO, abbreviated DMF-, dimethyl sulfoxide [(CH3)2SO, abbreviated DMSO-, and hexamethylphosphoramide {[(CH3)2N-3PO, abbreviated HMPA} are particularly useful. These solvents can dissolve many salts because of their high polarity, but they tend to solvate metal cations rather than nucleophilic anions. As a result, the bare, unsolvated anions have a greater nucleophilicity and SN2 reactions take place at correspondingly increased rates. For instance, a rate increase of 200,000 has been observed on changing from methanol to HMPA for the reaction of azide ion with 1-bromobutane.
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