Nucleophilic bimolecular substitution (SN2) occurs mainly with primary and sometimes secondary alkyl halides. Because of the geometry of the bimolecular mechanism, the reaction always takes place with inversion of configuration. A polar aprotic solvent like DMSO is used, because a protic solvent like water will over-stabilize the nucleophile through solvation and promote E1 elimination instead, or the SN1 mechanism. Both E2 and SN2 prefer polar aprotic solvents. With primary alkyl halides, however, regardless of solvent, the SN2 mechanism almost always predominates. This occurs even if the nucleophile is a strong Bronsted base. With primary alkyl halides, only strong, bulky (hindered) base like tert-butoxide can cause elimination (E2) to occur rather than SN2 substitution. The SN2 mechanism is more difficult to achieve with secondary alkyl halides than with primary. Instead of substitution, strong bases react by elimination (E2) with secondary alkyl halides. SN2 substitution is possible with secondary alkyl halides if the solvent is polar and aprotic and the nucleophile is a weak base, like cyanide ion or alcohol. SN2 substitution does not occur with tertiary alkyl halides, which are too hindered for nucleophilic attack, and it does not occur with vinylic or aryl halides.
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