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SN2 - Second-order Nucleophilic Substitution

Nucleophiles in SN2 Reactions

In SN reactions, the nucleophile always competes with the leaving group. If the potential leaving group is actually a better nucleophile than the attacking nucleophile, the SN reaction hardly occurs. Nucleophiles are often anions, whose lithium, sodium or potassium salts are usually applied in SN reactions. Some examples that are representative of such nucleophiles are:

  • Metal alkoxides: The alkoxide anion is the nucleophile. Alkoxides are good nucleophiles.
Fig.1
  • Metal carboxylates: The carboxylate anion is the nucelophile. However, carboxylates are relatively poor nucleophiles.
Fig.2
  • Thiolates, or metal mercaptans: The anionic sulphide functionality is a good nucleophile.
Fig.3
  • Metal cyanides: The cyanide anion is a particularly good nucleophile.
Fig.4

Electrically neutral molecules that contain a lone electron pair, such as amines or water, may also react as a nucleophile. A nucleophile always possesses a lone electron pair. Due to this lone electron pair, a nucleophile may also react as a base. However, the term "nucleophilicity" is not synonymous to the term "basicity"! If a nucleophile reacts as a base and not as a nucleophile, an elimination occurs instead of a substitution reaction. The ratio of elimination and substitution products is determined by the competition of elimination and substitution. For a detailed explanation of this subject refer to "Competition of Substitution and Elimination".

Competition of basicity and nucleophilicity

Nucleophilicity and basicity
Nucleophilicity is a kinetic measure that depicts the influence of a nucleophile on the reaction rate, while basicity is a thermodynamic measure that describes the influence of a base on the equilibrium position.

Exercise

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