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SN2 - Second-order Nucleophilic Substitution

SN Reactions

Alkyl halides (haloalkanes) frequently react with compounds that possess a lone electron pair (nucleophile), such as amines or OH. This reactivity may be explained by the uneven distribution of electron density between the carbon and the halogen in alkyl halides. As a result, nucleophiles attack the partially positively-charged electrophilic carbon and substitute the halogen (leaving group or nucleofuge). In the concluding analysis, a bond between the alkyl halide's carbon and the attacking nucleophile has been formed, while the initial carbon-halogen bond has been broken.

SN reactions
Reactions in which a substituent is exchanged by a nucleophile through an attack on an electrophilic atom of a substrate are called nucleophilic substitutions, or SN reactions.
Fig.1
Nucleophilic substitution.

SN reactions occur not only with alkyl halides but also with many other compound classes. They are equilibrium reactions. The equilibrium concentrations mainly depend on the characters of the nucleophile, the leaving group and the solvent.

SN reactions are classified by chemical kinetics:

In an SN2 reaction, the reaction rate depends on the concentrations of both starting products, as well as that of the substrate and nucleophile. In contrast, the reaction rate of an SN1 reaction is influenced only by the substrate's (alkyl halide's), and not the nucleophilie's, concentration.

The terms "SN1" and "SN2"
The "1" and "2" in the terms "SN1" and "SN2" represent the molecularity of the nucleohilic substitution. That is, the rate-determining step in SN1 reactions is unimolecular, while it is bimolecular in SN2 reactions.
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