Editing Chemical reaction (section)

===Substitution===
In a [[substitution reaction]], a [[functional group]] in a particular [[chemical compound]] is replaced by another group.<ref name=jerry>{{JerryMarch}}</ref> These reactions can be distinguished by the type of substituting species into a [[nucleophilic substitution|nucleophilic]], [[electrophilic substitution|electrophilic]] or [[radical substitution]].
{{multiple image | direction = vertical | image1 = SN1 reaction mechanism.png|width1 = 300|image2 = SN2 reaction mechanism.png|width2 = 300| caption1 = S<sub>N</sub>1 mechanism| caption2 = S<sub>N</sub>2 mechanism}}

In the first type, a [[nucleophile]], an atom or molecule with an excess of electrons and thus a negative charge or [[partial charge]], replaces another atom or part of the "substrate" molecule. The electron pair from the nucleophile attacks the substrate forming a new bond, while the [[leaving group]] departs with an electron pair. The nucleophile may be electrically neutral or negatively charged, whereas the substrate is typically neutral or positively charged. Examples of nucleophiles are [[hydroxide]] ion, [[alkoxide]]s, [[amine]]s and [[halide]]s. This type of reaction is found mainly in [[aliphatic hydrocarbon]]s, and rarely in [[aromatic hydrocarbon]]. The latter have high electron density and enter [[nucleophilic aromatic substitution]] only with very strong [[Polar effect|electron withdrawing groups]]. Nucleophilic substitution can take place by two different mechanisms, [[SN1 reaction|S<sub>N</sub>1]] and [[SN2 reaction|S<sub>N</sub>2]]. In their names, S stands for substitution, N for nucleophilic, and the number represents the [[order (chemistry)|kinetic order]] of the reaction, unimolecular or bimolecular.<ref>{{cite book | author = Hartshorn, S.R. | url = https://books.google.com/books?id=bAo4AAAAIAAJ | title = Aliphatic Nucleophilic Substitution | publisher = [[Cambridge University Press]] | location = London | year = 1973 | isbn = 978-0-521-09801-4 | page = 1}}</ref>
{{multiple image | direction = vertical
| align = right
| width = 120
| image1= Walden-inversion-3D-balls.png
|caption1=The three steps of an [[SN2 reaction|S<sub>N</sub>2 reaction]]. The nucleophile is green and the leaving group is red
|image2=SN2-Walden-before-and-after-horizontal-3D-balls.png
|caption2=S<sub>N</sub>2 reaction causes stereo inversion (Walden inversion)
}}

The S<sub>N</sub>1 reaction proceeds in two steps. First, the [[leaving group]] is eliminated creating a [[carbocation]]. This is followed by a rapid reaction with the nucleophile.<ref>{{Cite journal|author =  Bateman, Leslie C. | author2 = Church, Mervyn G. | author3 = Hughes, Edward D. | author4 = Ingold, Christopher K. | author5 = Taher, Nazeer Ahmed |doi = 10.1039/JR9400000979|title =  188. Mechanism of substitution at a saturated carbon atom. Part XXIII. A kinetic demonstration of the unimolecular solvolysis of alkyl halides. (Section E) a general discussion|year =  1940|journal =  Journal of the Chemical Society|page =  979}}</ref>

In the S<sub>N</sub>2 mechanisms, the nucleophile forms a transition state with the attacked molecule, and only then the leaving group is cleaved. These two mechanisms differ in the [[stereochemistry]] of the products. S<sub>N</sub>1 leads to the non-stereospecific addition and does not result in a chiral center, but rather in a set of [[Cis–trans isomerism|geometric isomers]] (''cis/trans''). In contrast, a reversal ([[Walden inversion]]) of the previously existing stereochemistry is observed in the S<sub>N</sub>2 mechanism.<ref>[[#Bruckner|Brückner]], pp. 63–77</ref>

[[Electrophilic substitution]] is the counterpart of the nucleophilic substitution in that the attacking atom or molecule, an [[electrophile]], has low electron density and thus a positive charge. Typical electrophiles are the carbon atom of [[carbonyl group]]s, carbocations or [[sulfur]] or [[nitronium]] cations. This reaction takes place almost exclusively in aromatic hydrocarbons, where it is called [[electrophilic aromatic substitution]]. The electrophile attack results in the so-called σ-complex, a transition state in which the aromatic system is abolished. Then, the leaving group, usually a proton, is split off and the aromaticity is restored. An alternative to aromatic substitution is electrophilic aliphatic substitution. It is similar to the nucleophilic aliphatic substitution and also has two major types, S<sub>E</sub>1 and S<sub>E</sub>2<ref>[[#Bruckner|Brückner]], pp. 203–206</ref>

[[File:Electrophilic aromatic substitution.svg|center|thumb|648px|Mechanism of electrophilic aromatic substitution]]
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In the third type of substitution reaction, radical substitution, the attacking particle is a [[Radical (chemistry)|radical]].<ref name=jerry/> This process usually takes the form of a [[chain reaction]], for example in the reaction of alkanes with halogens. In the first step, light or heat disintegrates the halogen-containing molecules producing radicals. Then the reaction proceeds as an avalanche until two radicals meet and recombine.<ref>[[#Bruckner|Brückner]], p. 16</ref>
:;<chem>X. + R-H -> X-H + R.</chem>
:;<chem>R. + X2 -> R-X + X.</chem>
::<small> Reactions during the chain reaction of radical substitution </small>