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Concept
SN2 reaction
Summary
DISPLAYTITLE:SN2 reaction
The SN2 reaction is a type of reaction mechanism that is common in organic chemistry. In this mechanism, one bond is broken and one bond is formed in a concerted way, i.e., in one step. The name SN2 refers to the Hughes-Ingold symbol of the mechanism: "SN" indicates that the reaction is a nucleophilic substitution, and "2" that it proceeds via a bi-molecular mechanism, which means both the reacting species are involved in the rate-determining step. The other major type of nucleophilic substitution is the SN1, but many other more specialized mechanisms describe substitution reactions.
The SN2 reaction can be considered as an analogue of the associative substitution in the field of inorganic chemistry.
The reaction most often occurs at an aliphatic sp3 carbon center with an electronegative, stable leaving group attached to it (often denoted X), which is frequently a halide atom. The breaking of the C–X bond and the formation of the new bond (often denoted C–Y or C–Nu) occur simultaneously through a transition state in which a carbon under nucleophilic attack is pentacoordinate, and approximately sp2 hybridised. The nucleophile attacks the carbon at 180° to the leaving group, since this provides the best overlap between the nucleophile's lone pair and the C–X σ* antibonding orbital. The leaving group is then pushed off the opposite side and the product is formed with inversion of the tetrahedral geometry at the central atom.
If the substrate under nucleophilic attack is chiral, then this often leads to inversion of configuration (stereochemistry), called a Walden inversion.
In an example of the SN2 reaction, the attack of Br− (the nucleophile) on an ethyl chloride (the electrophile) results in ethyl bromide, with chloride ejected as the leaving group.
If the molecule that is undergoing SN2 reaction has a chiral centre, then it is possible that the optical activity of the product would be different from that of the reactant. In an example, 1-bromo-1-fluoroethane can undergo SN2 reaction to form 1-fluoroethan-1-ol, with the nucleophile being an OH− group.
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