Editing SN2 reaction (section)

==Reaction mechanism==
The reaction most often occurs at an [[aliphatic]] [[orbital hybridisation|sp<sup>3</sup>]] carbon center with an [[electronegative]], stable leaving group attached to it, which is frequently a [[halogen]] (often denoted X). The formation of the C–Nu bond, due to attack by the nucleophile (denoted Nu), occurs together with the breakage of the C–X bond. The reaction occurs through a [[transition state]] in which the reaction center is [[coordination number|pentacoordinate]] and approximately sp<sup>2</sup>-hybridised.
[[File:SN2 reaction.svg|center|450px]]

The S<sub>N</sub>2 reaction can be viewed as a [[HOMO and LUMO|HOMO–LUMO interaction]] between the nucleophile and substrate. The reaction occurs only when the occupied lone pair orbital of the nucleophile donates electrons to the unfilled [[Antibonding molecular orbital|σ* antibonding orbital]] between the central carbon and the [[leaving group]]. Throughout the course of the reaction, a p orbital forms at the reaction center as the result of the transition from the [[molecular orbitals]] of the reactants to those of the products.<ref name="Clayden-2012">{{cite book |last1=Clayden |first1=Jonathan |last2=Greeves |first2=Nick |last3=Warren |first3=Stuart |title=Organic chemistry |date=2012 |publisher=Oxford University Press |location=Oxford |isbn=978-0-19-927029-3 |page=330 |edition=2nd}}</ref>
[[File:SN2 reaction orbitals and transition state.svg|center|550px]]

To achieve optimal orbital overlap, the nucleophile attacks 180° relative to the leaving group, resulting in the leaving group being pushed off the opposite side and the product formed with [[Inversion in a point|inversion]] of tetrahedral geometry at the central atom.

For example, the synthesis of macrocidin A, a fungal [[metabolite]], involves an intramolecular ring closing step via an S<sub>N</sub>2 reaction with a [[phenoxide]] group as the nucleophile and a halide as the leaving group, forming an [[ether]].<ref>{{cite journal|title=Synthesis of the Bioherbicidal Fungus Metabolite Macrocidin A|last1=Hasse|first1=Robert|last2=Schobert|first2=Rainer|date=November 28, 2016|url=https://doi.org/10.1021/acs.orglett.6b03240|journal=[[Organic Letters]]|volume=18|issue=24|pages=6352–6355|doi=10.1021/acs.orglett.6b03240|access-date=December 30, 2023|url-access=subscription}}</ref> Reactions such as this, with an alkoxide as the nucleophile, are known as the [[Williamson ether synthesis]].
[[File:Macrocidin A intramolecular etherification.svg|center|Synthesis of macrocidin A via S<sub>N</sub>2 etherification.]]

If the substrate that is undergoing S<sub>N</sub>2 reaction has a [[Stereocenter|chiral centre]], then inversion of [[Molecular configuration|configuration]] ([[stereochemistry]] and [[optical activity]]) may occur; this is called the [[Walden inversion]]. For example, 1-bromo-1-fluoroethane can undergo nucleophilic attack to form 1-fluoroethan-1-ol, with the nucleophile being an HO<sup>−</sup> group. In this case, if the reactant is levorotatory, then the product would be dextrorotatory, and vice versa.<ref>{{Cite book|last=CURTIS|first=CLIFF. MURGATROYD, JASON. SCOTT, DAVE|url=https://www.worldcat.org/oclc/1084791738|title=Edexcel international a level chemistry student book.|date=2019|publisher=EDEXCEL Limited|isbn=978-1-292-24472-3|location=[Place of publication not identified]|oclc=1084791738}}</ref>
[[File:SN2 Walden inversion example.svg|500px|center|S<sub>N</sub>2 mechanism of 1-bromo-1-fluoroethane with one of the carbon atoms being a chiral centre.]]