Editing SN2 reaction (section)

===Leaving group===
Good leaving groups on the substrate lead to faster S<sub>N</sub>2 reactions. A good leaving group must be able to stabilize the [[electron density]] that comes from breaking its bond with the carbon center. This leaving group ability trend corresponds well to the [[Acid dissociation constant|p''K''<sub>a</sub>]] of the leaving group's conjugate acid (p''K''<sub>aH</sub>); the lower its p''K''<sub>aH</sub> value, the faster the leaving group is displaced.

Leaving groups that are neutral, such as [[water]], [[alcohols]] ({{chem2|R\sOH}}), and [[amines]] ({{chem2|R\sNH2}}), are good examples because of their positive charge when bonded to the carbon center prior to nucleophilic attack. Halides ([[Chloride|{{chem2|Cl-}}]], [[Bromide|{{chem2|Br-}}]], and [[Iodide|{{chem2|I-}}]], with the exception of [[Fluoride|{{chem2|F-}}]]), serve as good anionic leaving groups because electronegativity stabilizes additional electron density; the fluoride exception is due to its strong bond to carbon.

Leaving group reactivity of alcohols can be increased with [[sulfonates]], such as [[Tosyl group|tosylate]] ({{chem2|-OTs}}), [[triflate]] ({{chem2|-OTf}}), and [[mesylate]] ({{chem2|-OMs}}). Poor leaving groups include [[hydroxide]] ({{chem2|-OH}}), [[alkoxides]] ({{chem2|-OR}}), and [[Azanide|amides]] ({{chem2|-NR2}}).
[[File:Alcohol to tosylate.svg|center|450px]]

The [[Finkelstein reaction]] is one S<sub>N</sub>2 reaction in which the leaving group can also act as a nucleophile. In this reaction, the substrate has a halogen atom exchanged with another halogen. As the negative charge is more-or-less stabilized on both halides, the reaction occurs at equilibrium.
[[File:Finkelstein reaction example.svg|center|400px]]