page_title: SN2 reaction
section_title: Reaction mechanism
hierarchical_section_title: SN2 reaction / Reaction mechanism
caption_attribution_description: English: new example of SN2 rxn, perhaps simpler
context_page_description: 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 synchronously, i.e., in one step. SN2 is a kind of nucleophilic substitution reaction mechanism, the name referring to the Hughes-Ingold symbol of the mechanism. Since two reacting species are involved in the slow step, this leads to the term substitution nucleophilic or SN2; the other major kind is SN1. Many other more specialized mechanisms describe substitution reactions.
  The reaction type is so common that it has other names, e.g. "bimolecular nucleophilic substitution", or, among inorganic chemists, "associative substitution" or "interchange mechanism".
context_section_description: 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.
  SN2 attack occurs if the backside route of attack is not sterically hindered by substituents on the substrate. Therefore, this mechanism usually occurs at unhindered primary and secondary carbon centres. If there is steric crowding on the substrate near the leaving group, such as at a tertiary carbon centre, the substitution will involve an SN1 rather than an SN2 mechanism, (an SN1 would also be more likely in this case because a sufficiently stable carbocation intermediary could be formed).
caption: SN2 reaction of chloroethane with bromide ion
exif: {}